(en)A gene encoding glutathione synthetase from Candida utilis is provided and food containing γ-glutamylcysteine cysteine or cystenylglycine is produced by cultivating Candida utilis modified by means of a gene encoding glutathione synthetase under a suitable condition and mixing the obtained culture or a fraction thereof or the culture or a fraction thereof subjected to heat treatment with a raw material of food or drink to process food or drink.

1.ApplicationNumber: US-91536604-A
1.PublishNumber: US-7297513-B2
2.Date Publish: 20071120
3.Inventor: NISHIUCHI HIROAKI
NISHIMURA YASUSHI

4.Inventor Harmonized: NISHIUCHI HIROAKI(JP)
NISHIMURA YASUSHI(JP)

5.Country: US
6.Claims:
(en)A gene encoding glutathione synthetase from Candida utilis is provided and food containing γ-glutamylcysteine cysteine or cystenylglycine is produced by cultivating Candida utilis modified by means of a gene encoding glutathione synthetase under a suitable condition and mixing the obtained culture or a fraction thereof or the culture or a fraction thereof subjected to heat treatment with a raw material of food or drink to process food or drink.

7.Description:
(en)BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a gene encoding glutathione synthetase from Candida utilis and Candida utilis in which intracellular content of glutathione is increased or decreased by means of the same gene, and food utilizing the same Candida utilis . γ-glutamylcysteine and cysteine produced therefrom, or glutathione and cystenylglycine produced therefrom are useful in food production.
2. Description of the Related Art
Cysteine is used for the purpose of improving food flavor, taste etc. While a proteolysis method, a semisynthesis method and so forth are known as methods for producing cysteine, the proteolysis method and the semisynthesis method are mainly used at present. In order to use cysteine for improvement of flavor and taste of food, natural food material having a high content of cysteine is being required. However, few natural food materials of such kind have been known heretofore. Meanwhile, it has been reported that a food material having a high content of cysteine can be obtained when yeast extract containing γ-glutamylcysteine is heated or treated with an enzyme (WO00/30474).
Cystenylglycine is a dipeptide in which cysteine and glycine are bound with each other through peptide bond. It has been reported that material for enhancing flavor of meat can be obtained by heating cystenylglycine together with sugar. Although it has been known that cystenylglycine is produced by peptide synthesis method, it has hardly ever been known that it is produced from natural materials. On the other hand, it is reported that food material having a high content of cystenylglycine can be obtained by heat-treatment or enzyme-treatment of yeast extract containing glutathione (JP2001-321117A).
In Saccharomyces cerevisiae , γ-glutamylcysteine is synthesized by the reaction of γ-glutamylcysteine synthetase using cysteine and glutamic acid as substrates. Further, glutathione is synthesized by the reaction of glutathione synthetase using γ-glutamylcysteine and glycine as substrates. Yeasts having a high content of γ-glutamylcysteine have been reported in WO00/30474; Otake et al., Agri. Biol. Chem., 54 (12): 3145-3150 (1990); Chris et al., Molecular Biology of the Cell., 8, 1699-1707 (1997); Inoue et al., Biochimica et Biophysica Acta, 1395, 315-320 (1998) and so forth. However, all of these reports relate to studies using Saccharomyces cerevisiae , and there has been no report about studies using Candida utilis.
Candida utilis is an industrially important microorganism which can be used in production of biologically important substances including glutathione, some kind of amino acids and enzymes (Journal of Bacteriology, 1995, vol.177, No.24, p7171-7177). Candida utilis has characteristics that it takes most of its energy from the pentose phosphate cycle producing a pyridine base, and that it shows weaker catabolite repression, compared to Saccharomyces cerevisiae (Biotechnology, 3, 30 (1983)). Further, since Saccharomyces cerevisiae is usually used for research purpose, there have been few findings about Candida utilis . Under such circumstances, there has been no report about how Candida utilis synthesizes glutathione, and it has been only reported that a specific strain of Candida utilis obtained as a zinc resistant strain produced a large amount of glutathione at the temperature which is lower by 5° C. or more than the normal cultivation of yeast (JP03-18872B).
Thus, a gene encoding glutathione synthetase has not been reported in Candida utilis , and there have not been known methods of producing industrially-useful substances by controlling intracellular content of glutathione.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a gene encoding glutathone synthetase from Candida utilis , a Candida utilis having high contents of glutathione and/or γ-glutamylcysteine and extract thereof. Another object of the present invention is to provide food which contains glutathione, γ-glutamylcysteine, cysteine or cystenylglycine and a method for producing the same.
It has been reported that the growth of Candida utilis per unit saccharide is better than that of Saccharomyces cerevisiae (Biotechnology, vo.3, p30 (1983)). Further, since it does not produce ethanol as a byproduct under a strictly aerobic condition (Kondo et al. (J. Bacteriology, December, 1995, pp. 7171-7177)), there is less need of paying attention to the ethanol by production during the cultivation. Therefore, the inventors of the present invention considered that yeast extract produced by using Candida utilis having a high content of γ-glutamylcysteine would become less expensive than yeast extract produced by using Saccharomyces cerevisiae , and hence desirable for industrial production.
Based on such concepts, the inventor of the present invention assiduously studied in order to solve the above mentioned objects. As a result, they succeeded in cloning a gene encoding glutathione synthetase and a gene encoding γ-glutamylcysteine synthetase from Candida utilis , based on the homology with the enzymes originated from other organisms. They found that Candida utilis which has been modified by means of the genes has high contents of γ-glutamylcysteine, and thereby accomplished the present invention.
The present invention essentially provides followings.
(1) A DNA which encodes a protein defined in the following (A) or (B):

(A) a protein which has the amino acid sequence of SEQ ID NO: 2
(B) a protein which has the amino acid sequence of SEQ ID NO: 2 including substitution, deletion, insertion or addition of one or several amino acids, and has glutathione synthetase activity.
(2) The DNA according to (1), which is defined in the following (a) or (b):

(a) a DNA which comprises the nucleotide sequence of the nucleotide numbers from 58 to 1485 of SEQ ID NO: 1;
(b) a DNA which is hybridizable with the DNA comprising the sequence of the nucleotide numbers from 58 to 1485 of SEQ ID NO: 1 or a probe that can be prepared from said nucleotide sequence under stringent conditions, and encodes a protein having glutathione synthetase activity.
(3) A Candida utilis which has been modified to have glutathione synthetase activity of 0.003 μmol GSH/mg protein/hour or less by means of the DNA as defined in (1) or (2). (4) A Candida utilis which has been modified so that glutathione synthetase activity is enhanced by means of the DNA as defined in (1) or (2). (5) A Candida utilis which has been modified to have glutathione synthetase activity of 0.003 mmol GSH/mg protein/hour or less by means of the DNA as defined in the following (c) or (d):

(c) a DNA which comprises the nucleotide sequence of the nucleotide numbers from 58 to 981 of SEQ ID NO: 1
(d) a DNA which is hybridizable with the DNA comprising the sequence of the nucleotide numbers from 58 to 981 of SEQ ID NO: 1 or a probe that can be prepared from the said nucleotide sequence under stringent conditions.
(6) The Candida utilis according to any one of (3) to (5), wherein the Candida utilis has further been modified so that γ-glutamylcysteine synthetase activity is enhanced by means of the DNA of SEQ ID No.3. (7) Food or drink which comprises culture obtainable by cultivating the Candida utilis according to any one of (3) to (6) under a suitable condition, a fraction of said culture containing glutathione and/or γ-glutamylcysteine, or said culture or fraction in which glutamic acid at gamma position of glutathione and/or γ-glutamylcysteine is released by heat-treatment or enzyme-treatment. (8) Yeast extract produced by using culture obtainable by cultivating the Candida utilis according to any one of (3) to (6) under a suitable condition. (9) A method for producing food containing γ-glutamylcysteine or cysteine, which comprises cultivating the Candida utilis according to any one of (3) to (6) under a suitable condition and mixing the obtained culture or a fraction thereof or the culture or a fraction thereof subjected to heat treatment with a raw material of food or drink to process food or drink.




BRIEF EXPLANATIN OF THE DRAWINGS
FIG. 1 shows construction of the cassette for substituting γ-glutamylcysteine synthetase gene of Saccharomyces cerevisiae.
FIG. 2 shows construction of Saccharomyces cerevisiae Nα4 strain.
FIG. 3 shows construction of the expression vector for glutathione synthetase gene of Candida utilis.
FIG. 4 shows construction of the expression vector for γ-glutamylcysteine synthetase gene of Candida utilis.
FIG. 5 shows construction (first half) of the cassette for disrupting glutathione synthetase gene of Candida utilis.
FIG. 6 shows construction (latter half) of the cassette for disrupting glutathione synthetase gene of Candida utilis.



DETAILED DESCRIPTION OF THE INVENTION
Hereafter, the present invention will be explained in detail.
<1> Glutathione Synthetase Gene of Candida utilis
The DNA of the present invention is a DNA which encodes a protein defined in the following (A) or (B):
(A) a protein which has the amino acid sequence of SEQ ID NO: 2; (B) a protein which has an amino acid sequence of SEQ ID NO: 2 including substitution, deletion, insertion or addition of one or several amino acids, and has glutathione synthetase activity.

The term “glutathione synthetase activity” refers to an activity of catalyzing a reaction for producing glutathione from γ-glutamylcysteine and glycine.
The glutathione synthetase encoded by the DNA of the present invention may include substitution, deletion, insertion, addition or inversion of one or several amino acids at one or more sites in the amino acid sequence of SEQ ID NO: 2 so long as the aforementioned enzymatic activity is not impaired. Although the number of “several” amino acids referred to herein differs depending on position or type of amino acid residues in a three-dimensional structure of the protein, it may be specifically 2 to 25, preferably 2 to 12, more preferably 2 to 7.
For example, a DNA encoding the protein shown in (A) can be mentioned as a DNA (CGSH2 DNA) having nucleotide sequence of nucleotide numbers 58 to 1485 in the sequence of SEQ ID No.1 of the Sequence Listing. Further, a DNA encoding a protein substantially identical to the aforementioned glutathione synthetase can be obtained by modifying the CGSH2 DNA by, for example, site-directed mutagenesis so that amino acid residues at a specific site should include substitution, deletion, insertion, addition or inversion. Further, such a modified DNA as described above can also be obtained by a known mutagenesis treatment. Examples of the mutagenesis treatment include a method of treating CGSH2 DNA in vitro with hydroxylamine or the like and a method of treating a microorganism containing CGSH2 DNA, for example, a bacterium belonging to the genus Escherichia , with ultraviolet-ray irradiation or a mutagenesis agent used in usual mutagenesis such as N-methyl-N′-nitro-N-nitrosoguanidine (NG) or ethyl methane sulfonate (EMS).
Further, the mutations which cause the aforementioned substitution, deletion, insertion, addition, inversion or the like of amino acid residues include a naturally-occurring mutation or variation, for example, a mutation or variation attributable to a difference in strains of Candida utilis containing glutathione synthetase.
The DNA encoding a protein substantially identical to glutathione synthetase-can be obtained by expressing a DNA having such a mutation as described above in a suitable cell of Saccharomyces cerevisiae or the like and examining the glutathione synthetase activity in the cell.
Further, a DNA which is hybridizable with a probe having the nucleotide sequence of CGSH2 DNA or a part thereof under stringent conditions and encodes a protein having a glutathione synthetase activity, or a DNA having homology of not less than 90%, preferably not less than 95%, more preferably not less than 99% with the nucleotide sequence of CGSH2 DNA and encodes a protein having a glutathione synthetase activity is included in the DNA of the present invention. The “stringent conditions” referred to herein are conditions under which a so-called specific hybrid is formed, and a non-specific hybrid is not formed. It is difficult to clearly express these conditions by using any numerical value. However, for example, the stringent conditions include conditions under which DNAs having high homology, for example, DNAs having homology of 75% or more, preferably 85% or more, more preferably 95% or more, hybridize with each other, but DNAs having homology lower than the above value do not hybridize with each other. More specifically, the stringent conditions include conditions under which DNAs hybridize with each other at a salt concentration corresponding to an ordinary condition of washing in Southern hybridization, i.e., 1×SSC, 0.1% SDS, preferably 0.1×SSC, 0.1% SDS, at 60° C.
A part of the nucleotide sequence of CGSH2 DNA can also be used as a probe. Such a probe can be prepared by PCR using oligonucleotides prepared based on the nucleotide sequence of CGSH2 DNA as primers and a DNA fragment containing the nucleotide sequence of CGSH2 DNA as a template. When a DNA fragment having a length of about 300 bp is used as the probe, a condition of washing in hybridization may be, for example, 2×SSC, 0.1% SDS, at 50° C.
The DNAs hybridize under the aforementioned conditions include those in which a stop codon is generated or those which are deficient in the activity due to a mutation. However, such DNAs can be removed by examining the enzymatic activity of the expressed product.
The CGSH2 DNA was confirmed to encode glutathione synthetase, since glutathione synthesis was accelerated when it was introduced into a strain of Saccharomyces cerevisiae whose glutathione synthetase had been weakened, as shown in the Examples described later.
As a result of homology search of the amino acid sequence of SEQ ID NO: 2 in a database, it showed homologies of 48% with the amino acid sequences of glutathione synthetases of Saccharomyces cerevisiae.
<2> Candida utilis of the Present Invention
Candida utilis of the present invention is a strain modified so that glutathione synthetase activity is increased or decreased by means of full length or partial fragment of the DNA of the present invention, and preferably further modified so that γ-glutamylcysteine synthetase activity is increased by means of the DNA of SEQ ID No.3. A DNA used to modify Candida utilis so that glutathione synthetase activity is decreased compared to a wild type strain may be the DNA having nucleotide sequence of nucleotide numbers 58 to 981 in the sequence of SEQ ID No.1, or a DNA which is hybridizable with the DNA having nucleotide sequence of nucleotide numbers 58 to 981 in the sequence of SEQ ID No.1 or a probe prepared from the nucleotide sequence under stringent conditions.
The Candida utilis of the present invention which has been modified so that glutathione synthetase activity is decreased is preferably Candida utilis in which glutathione synthetase activity is decreased compared to a parent strain and, for example, it preferably shows a glutathione synthetase activity of 0.003 μmol GSH/mg protein/hour or less, more preferably 0.001 μmol GSH/mg protein/hour or less (“GSH” represents glutathione). It is further preferred that the glutathione synthetase activity is below the detectable limit.
The Candida utilis of the present invention which has been modified so that glutathione synthetase activity is increased is preferably a Candida utilis in which glutathione synthetase activity is increased compared to a parent strain and, for example, it preferably shows a glutathione synthetase activity of not less than 0.450 μmol GSH/mg protein/hour, more preferably not less than 0.600 μmol GSH/mg protein/hour. The glutathione synthetase activity can be measured by the method of Gushima et al. (T. Gushima et al., J. Appl. Biochem., 5, 210 (1983)).
The Candida utilis of the present invention is obtainable by modifying an appropriate strain, for example, a wild type strain of Candida utilis , by gene recombination techniques (for example, the techniques disclosed in the following publications can be utilized: FEMS Microbiology Letters, 165, 335-340 (1998); J. Bacteriology, December 1995, pp. 7171-7177; Curr. Genet. 10 (8): 573-578 (1986); WO98/14600) so that the intracellular glutathione synthetase activity should be increased or reduced. The reduction of the glutathione synthetase activity includes loss of the glutathione synthetase-activity.
Further, as a method for reducing the glutathione synthetase activity by utilizing a gene recombination technique, there can be mentioned a method of modifying a gene encoding glutathione synthetase so that the glutathione synthetase activity should be reduced. A nucleotide sequence of the gene encoding glutathione synthetase of the Candida utilis ATCC15239 strain is shown as SEQ ID NO: 2. Any glutathione synthetase gene from yeast of the genus Candida had not been previously known. The inventors of the present invention searched amino acid sequences of glutathione synthetase of various organisms for a highly conserved region and found the regions of SEQ ID NOS: 5 to 11. Then, they successfully amplified a gene fragment that was expected to encode glutathione synthetase from chromosomal DNA of Candida utilis by performing PCR using primers corresponding to the amino acid sequences of SEQ ID NOS: 6, 10 and 11 among the aforementioned regions. They succeeded in isolating full-length gene encoding glutathione synthetase of Candida utilis by performing 5′-RACE and 3′-RACE with the fragment. As the primers for obtaining the full-length gene, the primers of SEQ ID NOS: 12 to 19 can be mentioned. Further, although a strain of Candida utilis is not particularly limited, for example, the ATCC 15239 strain can be mentioned. This strain is obtainable from American Type Culture Collection (Address: 10801 University Boulevard, Manassas, Va. 20110-2209, United States of America).
As the method for reducing the glutathione synthetase activity by modifying a gene encoding glutathione synthetase, for example, there can be mentioned a method of modifying an expression regulatory sequence of the gene so that the expression amount of glutathione synthetase should be reduced, or modifying the coding region so that a protein having glutathione synthetase activity should not be expressed. Specifically, for example, the gene on the chromosome can be disrupted by transforming Candida utilis with recombinant DNA containing a mutant glutathione synthetase gene of which 5′- and 3′-ends are deleted and causing recombination between the mutant gene and the wild-type gene on the chromosome. In this method, operations become easier if a marker gene is contained in the recombinant DNA which can be selected according to a characteristic of a host such as auxotrophy. Further, if the recombinant DNA is linearized beforehand by digestion with a restriction enzyme or the like, a strain in which the recombinant DNA is incorporated into the chromosome is efficiently obtained.
Further, the gene on the chromosome can also be disrupted by introducing a recombinant DNA containing a mutant gene which has been modified so as not to produce glutathione synthetase with normal function by deleting a part of the glutathione synthetase gene, into Candida utilis and causing recombination between the mutant gene and the normal gene on the chromosome.
In the strain in which the recombinant DNA is incorporated into the chromosome as described above, recombination is caused between the introduced DNA and a glutathione synthetase gene that originally exists on the chromosome, and thereby two of fusion genes of the wild-type glutathione synthetase gene and the mutant glutathione synthetase gene are inserted into the chromosome so as to sandwich the other portions of the recombinant DNA (vector portion and marker gene). Therefore, the wild-type glutathione synthetase gene still functions in this state.
Subsequently, in-order to leave only the mutant glutathione synthetase gene on the chromosomal DNA, one copy of the glutathione synthetase gene is eliminated from the chromosomal DNA together with the vector portion (containing the marker gene) by recombination of two of the glutathione synthetase genes. At this step, the wild-type glutathione synthetase gene is left on the chromosomal DNA and the mutant glutathione synthetase gene is excised, or alternatively, the mutant glutathione synthetase gene is left on the chromosomal DNA and the wild-type glutathione synthetase gene is excised. Since the marker gene is eliminated in either case, occurrence of the second recombination can be confirmed based on a phenotype corresponding to the marker gene. Further, a target gene-substituted strain can be selected by amplifying the glutathione synthetase gene by PCR and examining its structure.
As the glutathione synthetase gene or a fragment thereof used for the gene disruption, in addition to the DNA having the nucleotide sequence of SEQ ID NO: 1, there can be mentioned a DNA which is hybridizable with the DNA under stringent conditions and or the DNA having homology of 90% or more, preferably 95% or more, more preferably 99% or more, with the nucleotide sequence of SEQ ID NO: 1. The stringent conditions include conditions under which DNAs hybridize with each other at a salt concentration corresponding to ordinary conditions of washing in Southern hybridization, i.e., 1×SSC, 0.1% SDS, preferably 0.1×SSC, 0.1% SDS, at 60° C.
The disruption of the glutathione synthetase gene of Saccharomyces cerevisiae is disclosed in WO00/30474. Examples of the methods for introducing the recombinant DNA into Candida utilis include the electroporation method (Luis et al., FEMS Microbiology Letters, 165, 335-340 (1998)).
The strain in which glutathione synthase activity has been reduced can be selected by using sensitivity to methylglyoxal as phenotype (Y. Ohtake et al., Agri. Biol. Chem., 54 (12): 3145-3150 (1990)). A strain that can synthesize a certain amount of glutathione (a strain having the glutathione synthetase activity and γ-glutamylcysteine synthetase activity) exhibits resistance to methylglyoxal. Further, the reduction of the glutathione synthetase activity can also be confirmed by examining the growth in a medium not containing glutathione. Further, a strain having a reduced glutathione synthetase activity is efficiently obtainable by utilizing MNNG (N-methyl-N′-nitro-N-nitrosoguanidine) concentration gradient plate (WO03/046154).
On the other hand, as a method for enhancing glutathione synthetase activity using gene recombination technique, there can be exemplified a method for modifying the glutathione synthetase gene so that glutathione synthetase activity of the protein encoded by the gene is enhanced or a method for modifying the glutathione synthetase gene so that expression amount of the gene is enhanced. For example, glutathione synthetase activity can be enhanced by introducing the glutathione synthetase having nucleotide sequence of SEQ ID No: 1 into Candida utilis in an expressible form.
A method for introducing the glutathione synthetase gene into Candida utilis includes a method in which Candida utilis is transformed with a recombinant DNA containing the glutathione synthetase gene and a DNA sequence existing on a Candida utilis chromosome to incorporate the recombinant DNA into chromosome of Candida utilis (Kondo, K. et al., J. Bacteriol., 1995, 177, p7171-7177). Specifically, introduction of DNA can be performed in a similar method as the above-mentioned gene substitution.
The Candida utilis of the present invention may have an enhanced γ-glutamylcysteine synthetase activity in addition to the modified glutathione synthetase activity. The γ-glutamylcysteine synthetase activity can be enhanced by introducing the γ-glutamylcysteine synthetase gene into Candida utilis in an expressible form. Examples of the γ-glutamylcysteine synthetase gene include the gene derived from Candida utilis as shown in SEQ ID No.3.
Examples of the method for introducing the γ-glutamylcysteine synthetase gene into Candida utilis include, for example, a method of transforming Candida utilis with recombinant DNA containing this gene and a DNA sequence existing on the Candida utilis chromosome to incorporate the recombinant DNA into the chromosome (K. Kondo et al., J. Bacteriol., 177, 7171-7177 (1995)). Specifically, the introduction can be performed in a similar manner as the aforementioned gene substitution.
Further, a target gene can also be introduced into Candida utilis by using a plasmid containing an autonomously replicable sequence (ARS) existing on the chromosomal DNA. The ARS of Candida utilis and the transformation using the same are described in WO95/32289.
<3> Yeast Extract, Food and Drink of the Present Invention and Method for Producing the Same
The culture obtained by cultivating the yeast in which glutathione synthetase activity has been modified as described above under a suitable condition, or a fraction thereof contains glutathione or γ-glutamylcysteine. The culture may be a culture broth containing yeast cells, yeast cells, disrupted cells or cell extract (yeast extract) obtained from the culture. A fraction containing glutathione and γ-glutamylcysteine may be obtainable from disrupted cells or yeast extract.
γ-glutamylcysteine is decomposed into cysteine and pyrrolidonecarboxylic acid when the aforementioned culture containing γ-glutamylcysteine or a fraction thereof is heated, and thereby, cysteine can be released. Specifically, cysteine can be produced by keeping the culture or a fraction thereof in an acidic to neutral condition, specifically at pH 1 to 7, at 50 to 120° C. for 3 to 300 hours in the presence of water.
Further, cysteine can also be produced by adjusting the culture containing γ-glutamylcysteine or a fraction thereof to pH 3 to 9, adding a γ-glutamyl peptide-decomposing enzyme (γ-glutamyltransferase, γ-glutamylcyclotransferase, glutaminase etc.) and allowing it to act on the γ-glutamylcysteine at 15 to 70° C. for 1 to 300 minutes.
Glutathione is decomposed into cystenylglycine and pyrrolidonecarboxylic acid when the aforementioned culture containing glutathione or a fraction thereof is heated, and thereby, cystenylglycine can be released. Specifically, cystenylglycine can be produced by keeping the culture or a fraction thereof in an acidic to neutral condition, specifically at pH 1 to 7, at 50 to 120° C. for 3 to 300 hours in the presence of water.
Further, cystenylglycine can also be produced by adjusting the culture containing glutathione or a fraction thereof to pH 3 to 9, adding a γ-glutamyl peptide decomposing enzyme (γ-glutamyltransferase, γ-glutamylcyclotransferase, glutaminase etc.) and allowing it to act on the glutathione at 15 to 70° C. for 1 to 300 minutes.
The medium used for the cultivation is not particularly limited so long as the yeast of the present invention grows well and glutathione or γ-glutamylcysteine is efficiently produced. If required, necessary nutrients are added to the medium depending on the characteristics of the yeast.
The cultivating conditions, the preparation of the yeast extract and the like can be performed in the same manner as usual culture of yeast or usual preparation of yeast extract. The yeast extract may be produced by extracting the yeast cells with hot water or by digesting yeast cells. Further, the yeast extract of the present invention may be extract in which cysteine or cystenylglycine is generated by heat treatment or enzymatic treatment, or may be extract which is subjected to heat treatment when or after it is processed into food or drink with other raw materials of the food or drink.
Specifically, the heat treatment of the yeast extract can be performed as follows. Water is added to yeast extract powder, and the mixture is adjusted to pH 5 with hydrochloric acid to prepare an aqueous solution with a concentration of 10%. Then, this solution is heated at 98° C. for 180 minutes.
The culture containing γ-glutamylcysteine, cysteine, glutathione or cystenylglycine or a fraction thereof can be used for production of food or drink. As examples of the food or drink, alcohol drink, breads, and fermented food seasonings can be mentioned. Production of cysteine from γ-glutamylcysteine by heat treatment and production of cystenylglycine from glutathione by heat treatment may be performed during or after the production of the food or drink. Alternatively, prior to the production of the food or drink, the yeast culture or a fraction thereof may be subjected to heat treatment.
The aforementioned food or drink is produced by mixing a culture containing γ-glutamylcysteine, cysteine, glutathione or cystenylglycine or a fraction thereof with raw materials of the food or drink and processing the mixture into the food or drink. The food or drink of the present invention can be produced by using the same raw materials as those of usual food or drink according to a similar method except that the aforementioned culture or the fraction is used. Examples of such raw materials include rice, barley, corn starch and so forth for alcohol drinks, wheat flower, sugar, salt, butter, yeast for fermentation and so forth for breads, and soybean, wheat and so forth for fermented food seasonings. Further, yeast extract or its concentrate, or dried products thereof can be used as a fermented food seasoning.
EXAMPLES
The present invention will be explained more specifically with reference to the following examples.
<1> Acquisition of a Gene Deduced to Encode Glutathione Synthetase of Candida utilis
The Candida utilis ATCC15239 strain was cultivated at 30° C. with shaking in a YPD test tube medium, and chromosome was prepared from the cells by using Dr. GenTLE for Yeast (Takara Shuzo Co., Code 9084).
[Composition of YPD Medium]











Glucose
2%


Peptone
1%


Yeast extract
1%


(pH 5.0)


















The following sequences were selected showing high homology between amino acid sequences of glutathione synthetases of Saccharomyces cerevisiae, Schizosaccharomyces pombe and rat (for these, Chris et al., Molecular Biology of the Cell., 8, 1699-1707 (1997)).








(i)
QEVAVVYYR
(SEQ ID NO: 5)




(ii)
GSKKIQQ
(SEQ ID NO: 6)



(iii)
VLKPOREGGGNNVLKPQREGGGNN
(SEQ ID NO: 7)



(iv)
ISELGIYG
(SEQ ID NO: 8)



(v)
GGVAAGF
(SEQ ID NO: 9)
















Degenerated primers were designed based on these amino acid sequences, and degenerated PCR was performed by using degenerated primers corresponding to each pair of (i) and (ii), (i) and (iii), (i) and (iv), (i) and (v), (ii) and (iii), (ii) and (iv), (ii) and (v), (iii) and (iv), (iii) and (v), and (iv) and (v). Each PCR product was subjected to agarose gel electrophoresis, and a region for the expected size was excised to prepare DNA. Further, although nested PCR was performed by using this prepared DNA as a template, a target fragment could not be obtained.
Subsequently, primers corresponding to the following amino acid sequences were designed with reference to the frequency of codons used in Candida utilis .










(vi)
GSKKIQQ
(SEQ ID NO: 6






(vii)
EGGGNN
(SEQ ID NO: 10)





(viii)
PQREGGG
(SEQ ID NO: 11)












PCR was performed by using a primer corresponding to the amino acid sequence of (vi) (GGT TCY AAG AAG ATY CAR CA, SEQ ID NO: 12) and a primer corresponding to the amino acid sequence of (vii) (CCA CCA CCY TCT CTY TGT GG, SEQ ID NO: 13). PCR was performed by using KOD Dash (TOYOBO Co., Code LDP-101) according to the manufacturer's instruction under the conditions of a reaction at 94° C. for 2 minutes, followed by 22 cycles of reactions at 94° C. for 1 minute, 55° C. (lowering temperature by 0.5° C. at each cycle) for 1 minute and 74° C. for 40 seconds and 15 cycles of reactions at 94° C. for 1 minute, 50° C. for 1 minute and 74° C. for 40 seconds.
The PCR product was subjected to agarose gel electrophoresis (Nusieve 3:1 agarose 3%, 1×TAE solution (Takara Shuzo Co., Code F5180A)). The gel was stained by using an ethidium bromide solution, then a region corresponding to 100 to 300 bp was excised, and DNA was extracted from the gel by using MagExtractor (TOYOBO Co., Code NPK-601).
When nested PCR was performed by using this DNA as a template, a primer corresponding to the region of (vi) (SEQ ID NO: 12) and a primer designed corresponding to the region of (viii) (GTT GTT ACC ACC ACC YTC, SEQ ID NO: 14), three bands were detected in the region corresponding to 100 to 300 bp. PCR was performed under the same condition as described above. These three bands were each excised, and each DNA was extracted from the gel. DNA of each band was ligated to a pGEM-T Easy vector (Promega Co.) by using DNA Ligation Kit Ver. 2 (Takara Shuzo Co.) and used to transform Escherichia coli JM109 competent cells (Takara Shuzo Co., Code 9052). Among the obtained transformants, one of transformant obtained was expected to contain a gene fragment expected to encode glutathione synthetase of Candida utilis . The nucleotide sequence of the insert contained in the transformant was determined in a conventional manner.
3′ Rapid amplification of cDNA ends (RACE) was performed based on the nucleotide sequence determined as described above by using 3′-RACE System for Rapid Amplification of cDNA Ends (Gibco BRL, Cat.No.18373-027). A cDNA primary strand was prepared from mRNA prepared from Candida utilis by using Rneasy Mini Kit (QIAGEN Co., Cat. No. 74104), and PCR was performed three times. The primers used are shown below.
First PCR: AAG ATA TAC CCA TTG GAT GG (SEQ ID NO: 15)
and AUAP primer attached to the 3′ RACE Kit

Second PCR: TCA GAT CTT GGT AAA GAG GC (SEQ ID NO: 16)
and AUAP primer attached to the 3′ RACE Kit

Third PCR: AGA CTG GCA TTT GAG TCT CC (SEQ ID NO: 17)
and AUAP primer attached to the 3′ RACE Kit

PCR was performed by using KOD Dash (TOYOBO Co., Code LDP-101) according to the manufacturer's instruction under the conditions of a reaction at 94° C. for 2 minutes, followed by 30 cycles of reactions at 94° C. for 1 minute, 50° C. for 30 seconds and 74° C. for 40 seconds.
The PCR product-was ligated to the pGEM-T Easy vector and used to transform Escherichia coli . The nucleotide sequence of the insert contained in the obtained transformant was analyzed to obtain information of a gene fragment expected to encode glutathione synthetase of Candida utilis.
Based on the nucleotide sequences as determined above, 5′ RACE was performed by using GeneRacer Kit (Invitrogen Co., Cat.No.L1502-01), however SuperSucript III RT (Invitrogen Co., Cat. No.18080-044) was used in reverse transcription in place of SuperSucript II of the kit. cDNA primary strand was synthesized from mRNA isolated by Rneasy mini kit (QIAGEN Co., Cat. No.74104) from Candida utilis , and then, nested PCR was performed.
The primers used are shown below.
Primer for preparing cDNA: TTG ACA CCA ACT CGG AGA CAT CAG A (SEQ ID NO: 18) Nested PCR: GAG AGT ACC TTC TCA TCC GTC AAG A, (SEQ ID NO: 19) and
GeneRacer 5′nested primer attached to the kit

The PCR product was ligated to the pCR4-TOPO vector in the kit according to TOPO TA cloning method and used to transform Escherichia coli . The nucleotide sequence of the insert contained in the transformant was analyzed, and the gene fragment deduced to contain the glutathione synthetase gene of Candida utilis was obtained.
Primers were designed based on the information obtained as described above, PCR was performed, and the nucleotide sequence of the amplification product was determined. PCR was performed by using Pyrobest (Takara Shuzo Co.) according to the manufacturer's instruction under the conditions of a reaction at 98° C. for 2 minutes, followed by 40 cycles of reactions at 98° C. for 20 seconds, 55° C. for 30 seconds and 72° C. for 2 minutes.
The primers used are shown below.









Primer F1:




TAG CCA ATA CAA CCA GCA ACA C
(SEQ ID NO: 20)





Primer R1:


GAA GGA ATT CTA ATT CGT GAG G
(SEQ ID NO: 21)












The nucleotide sequence of the PCR product amplified as described above was determined in a conventional manner, and it was found to be consistent with the nucleotide sequence shown in SEQ ID No: 1. The amino acid sequence deduced to be encoded by this nucleotide sequence is shown in SEQ ID NO: 2. Thereby, a homologue of the glutathione synthetase gene of Candida utilis was obtained.
<2> Acquisition of a Gene Deduced to Encode γ-Glutamylcysteine Synthetase of Candida utilis
The following sequences were selected showing high homology between the amino acid sequences of γ-glutamylcysteine synthetases of Saccharomyces cerevisiae and Schizosaccharomyces pombe (Ohtake et al., Yeast, 7 (9): 953-961 (December, 1991); Mutoh et al., J. Biochem. (Tokyo), 117 (2): 283-288 (February, 1995)).










(ix)
MGFGMG
(SEQ ID NO: 22)






(x)
GWRVEFR
(SEQ ID NO: 23)










Degenerated primers were designed based on each amino acid sequence, and degenerated PCR was performed. Primer F1 (ATG GGN TTY GGN ATG GG, SEQ ID NO: 24) was designed as a primer corresponding to the region of (iX), and primer R1 (RAA YTC NAC NCK CCA, SEQ ID NO: 25) was designed as a primer corresponding to the region of (X). PCR was performed by using KOD Dash (TOYOBO Co.) as DNA polymerase according to the manufacturer's instruction under the conditions of a reaction at 94° C. for 3 minutes, followed by 30 cycles of reactions at 94° C. for 1 minute, 52° C. for 1 minute and 74° C. for 1 minute.
The PCR product was subjected to agarose gel electrophoresis, a region corresponding to the expected size of about 700 bp was excised, and DNA was extracted from the gel by using MagExtractor (TOYOBO Co., Code NPK-601).
Further, nested PCR was performed by using this extracted DNA as a template. PCR was performed in the same manner as described above. The amplification product was subjected to agarose gel electrophoresis and then stained by using an ethidium bromide solution, a region corresponding to about 700 bp was excised, and DNA was extracted from the gel by using MagExtractor. The DNA was ligated to the pGEM-T Easy vector by using DNA Ligation Kit Ver. 2 (Takara Shuzo Co.) and used to transform Escherichia coli JM109 competent cells. Among the obtained transformants, one was obtained as a transformant considered to contain a gene fragment expected to encode γ-glutamylcysteine synthetase of Candida utilis . The nucleotide sequence of the insert contained in the transformant was determined in a conventional manner.
3′ RACE was performed based on the nucleotide sequence determined as described above by using 3′ RACE System for Rapid Amplification of cDNA Ends (Gibco BRL). A cDNA primary strand was synthesized from mRNA prepared from Candida utilis by using Rneasy Mini Kit, and PCR was performed 3 times.
The primers used are shown below.
First PCR: TGA ACA GAG CTC GTT ACC TC (SEQ ID NO: 26)
and AUAP primer attached to the 3′ RACE Kit

Second PCR: TCA TGG GCT AAT TTT GCA CC (SEQ ID NO: 27)
and AUAP primer attached to the 3′ RACE Kit

Third PCR: TTC CTA GCA TTG ACG GCA GC (SEQ ID NO: 28)
and AUAP primer attached to the 3′ RACE Kit

PCR was performed by using KOD Dash according to the manufacturer's instruction under the conditions of a reaction at 94° C. for 2 minutes, followed by 30 cycles of reactions at 94° C. for 1 minute, 50° C. for 30 seconds and 74° C. for 40 seconds. The PCR product was ligated to the PGEM-T Easy vector and used to transform Escherichia coli . The nucleotide sequence of the insert contained in the transformant was analyzed to obtain information about the gene fragment expected to contain the γ-glutamylcysteine synthetase gene of Candida utilis.
Subsequently, 5′ RACE was performed based on the nucleotide sequences as previously elucidated. The kit used was 5′ RACE System for Rapid Amplification of cDNA Ends Reagent Assembly Version 2.0 (Gibco BRL).
The primer used for RT (reverse transcription) for preparing a cDNA primary strand is shown below.









AGC ACC AGA AAT GAC GTT C
(SEQ ID NO: 29)









PCR was performed 3 times by using the cDNA library constructed according to the manufacturer's instruction and the following primers. PCR was performed by using KOD Dash according to the manufacturer's instruction under the conditions of a reaction at 94° C. for 2 minutes, followed by 30 cycles of reactions at 94° C. for 1 minute, 50° C. for 30 seconds and 74° C. for 40 seconds.
The primers used are shown below.
First PCR: CCA TCT GAC GAC ATC CTG CTG (SEQ ID NO: 30)
AUAP primer attached to the kit

Second PCR: GTC AGC TAA GTG GCC TTT G (SEQ ID NO: 31)
AUAP primer attached to the kit

Third PCR: CAC TGG CGC TGC TGC CGT C (SEQ ID NO: 32)
AUAP primer attached to the kit

The PCR product was ligated to the pGEM-T Easy vector and used to transform Escherichia coli . The nucleotide sequence of the insert contained in the transformant was analyzed to obtain information about the gene fragment expected to contain the γ-glutamylcysteine synthetase gene of Candida utilis . Based on homologies with those of other organisms, it was considered that the full length had not been cloned, and 5′ RACE was further performed.
The primer used for RT to prepare a cDNA primary strand is shown below.









TGA TCT TCT GCT GTT CAT GTT
(SEQ ID NO: 33)









PCR was performed 3 times by using the cDNA library constructed according to the manufacturer's instruction.
The primers used are shown below.
First PCR: CTC CAC GTA CAA GTA GTT CTC (SEQ ID NO: 34)
AUAP primer attached to the kit

Second PCR: CAG CGA ATC ACC GTT GTA CGG (SEQ ID NO: 35)
AUAP primer attached to the kit

Third PCR: AGC CAG CGG TGT CGC CTC (SEQ ID NO: 36)
primer attached to the kit

The PCR product was ligated to the pGEM-T Easy vector and used to transform Escherichia coli . The nucleotide sequence of the insert contained in the transformant was analyzed to obtain information about the gene fragment expected to contain the γ-glutamylcysteine synthetase gene of Candida utilis.
Primers were designed based on the information obtained as described above, PCR was performed, and the nucleotide sequence of the amplification product was determined. PCR was performed by using Pyrobest (Takara Shuzo Co.) according to the manufacturer's instruction under the conditions of a reaction at 98° C. for 2 minutes, followed by 40 cycles of reactions at 98° C. for 20 seconds, 55° C. for 30 seconds and 72° C. for 2 minutes.
The primers used are shown below.







Primer F2:



GGG TTT GTT GTC TAT CGG CTT AAG
(SEQ ID NO: 37)



Primer R2:

AGC TGT CTT GGT CGT CAT ATC CAT
(SEQ ID NO: 38)












The nucleotide sequence of the PCR product amplified as described above was determined in a conventional manner. The result is shown in SEQ ID NO: 3. Further, the amino acid sequence of γ-glutamylcysteine synthetase expected to be encoded by this nucleotide sequence is shown in SEQ ID NO: 4. Thus, a homologue of the γ-glutamylcysteine synthetase gene of Candida utilis was obtained.
The functions of the above-identified γ-glutamylcysteine synthetase gene and glutathione synthetase homologue gene derived from Candida utilis were determined by expressing them in Saccharomyces cerevisiae . Vectors for expressing each gene in Saccharomyces cerevisiae were constructed and the vectors were introduced into Saccharomyces cerevisiae , thereby the functions of the homologues were analyzed.
As Saccharomyces cerevisiae having a reduced glutathione synthetase activity, the Saccharomyces cerevisiae Nα3 strain described in WO01/90310 was used.
The Nα3 strain is a strain constructed by replacing glutathione synthetase gene with a gene encoding weakened glutathione synthetase, from the parent strain of Nα1 strain (WO01/90310) which is a haploid uracil-auxotrophic strain of Saccharomyces cerevisiae.
<3> Acquisition of Saccharomyces cerevisiae Having Reduced γ-Glutamylcysteine Synthetase Activity
(1) Production of GSH1 Gene Substitution Cassette
First, the fragment ranging from the middle region to the 3′-end of the γ-glutamylcysteine synthetase gene (SEQ ID NO: 39) was amplified by PCR using chromosomal DNA of the aforementioned Nα1 strain as a template. PCR was performed by using KOD Dash (TOYOBO Co.) and a reaction mixture having the following composition according to the manufacturer's instruction under the conditions of a reaction at 94° C. for 1 minute, followed by 30 cycles of reactions at 94° C. for 30 seconds, 60° C. for 40 seconds and 74° C. for 1 minute. As primers, GF1 (GTG GAC GAC CGT ACT CCG AAG, SEQ ID NO: 41) and GR1 (ACC CAA ATC GAT AAT GTC AAC, SEQ ID NO: 42) were used.
The GSH1 gene fragment amplified as described above was ligated to the plasmid pGEM-T Easy (Promega Co.) according to the manufacturer's instruction to obtain GSH1dash/pGEM.
Subsequently, by site-directed mutagenesis, codons corresponding to the amino acids of the 372nd and 373rd positions, namely, serine and lysine, of the γ-glutamylcysteine synthetase (SEQ ID NO: 40) were replaced with a stop codon in the γ-glutamylcysteine synthetase gene (SEQ ID NO: 39) contained in GSHldash/pGEM. This operation was performed by using Quick Change Site-Directed Mutagenesis Kit (STRATAGENE Co.) according to the protocol of the manufacturer. As primers, QCF1 (CTT TTC TTG GGT GGG TAG TAA TTT TTC AAT AGG ACT, SEQ ID NO: 43) and QCR1 (AGT CCT ATT GAA AAA TTA CTA CCC ACC CAA GAA AAG, SEQ ID NO: 44) were used. Thereby, the plasmid GSH1Mdash/pGEM was produced.
The γ-glutamylcysteine synthetase in which mutations have been introduced as described above has a weak enzymatic activity (weakened glutathione synthetase, WO01/90310).
Subsequently, the plasmids pYES2dash (a plasmid obtained by eliminating 2μ ori from the plasmid pYES2 (Invitrogen Co.)) described in WO01/90310 and the aforementioned GSH1Mdash/pGEM were both digested with restriction enzymes SacI and SphI. A fragment containing the URA3 gene was excised from pYES2dash, a region containing a partial gene sequence of γ-glutamylcysteine synthetase was excised from GSH1Mdash/pGEM, and these were ligated to each other to prepare a plasmid GSH1Mdash/pYES2dash. GSH1Mdash/pYES2dash was digested with a restriction enzyme BbeI to obtain a gene substitution cassette ( FIG. 1 ).
(2) Introduction of GSH1 Gene Substitution Cassette into Saccharomyces cerevisiae
Gene substitution of the γ-glutamylcysteine synthetase gene in the Nα1 strain was performed by using the gene substitution cassette produced as described above. The Nα1 strain was precultivated, and the culture was inoculated in 50 ml of YPD medium until the cells reached the logarithmic growth phase. The cultivated cells were suspended in 1 M sorbitol, mixed with the gene substitution cassette and transformed by electroporation. The transformant strains were spread on an SD plate containing 1 mM glutathione, and grown strains were selected. It was confirmed by PCR that the gene substitution cassette had been incorporated into the chromosome at the target position, and the obtained strain was designated as Nα4 intermediate. Subsequently, the following operation was performed to leave only the mutant γ-glutamylcysteine synthetase gene on the chromosome. The Nα4 intermediate was cultivated in a YPD medium containing 1 mM glutathione, and the broth culture was inoculated on an SDFOA plate containing 1 mM glutathione. The γ-glutamylcysteine synthetase gene of a strain grown on the plate was sequenced to confirm that the sequence at the target site was correctly substituted, and thus the Nα4 strain was obtained ( FIG. 2 ).
Subsequently, whether the γ-glutamylcysteine synthetase activity of the Nα4 strain obtained as described above was weakened or not was examined. Ohtake et al. measured the γ-glutamylcysteine synthetase activity of the YH1 strain obtained from the Saccharomyces cerevisiae YNN27 strain by mutagenesis treatment (Agric. Biol. Chem., 54 (12): 3145-3150 (1990)). The activity was measured according to this method. As a result, the γ-glutamylcysteine synthetase activity of the Nα4 strain was below the detectable limit. Then, the Nα4 strain was cultivated in an SD medium, and the contents of γ-glutamylcysteine and glutathione in the cells in the logarithmic growth phase were measured. However, γ-glutamylcysteine was not detected, and the concentration of glutathione was 0.01%. Further, since the Nα4 strain exhibited sensitivity to 2 mM methylglyoxal, it was confirmed that the γ-glutamylcysteine synthetase gene had been substituted as in the case of the YH1 strain.
<4> Construction of Expression Vector of Glutathione Synthetase Homologue Derived from Candida utilis
PCR was performed by using chromosomal DNA of Candida utilis as a template to amplify the ORF region of the glutathione synthetase gene homologue (CGSH2) of Candida utilis . PCR was performed by using Pyrobest (Takara Shuzo Co.) according to the manufacturer's instruction under the conditions of a reaction at 98° C. for 2 minutes, followed by 40 cycles of reactions at 98° C. for 20 seconds, 55° C. for 30 seconds and 72° C. for 2 minutes. PCR was performed by using a N-terminus primer CGSH2F1 (AGA TTG GGT ACC ATG AGT ATT CCT CAG TTA TCT G, SEQ ID NO: 45), to which a KpnI digestion site was added, and a C-terminus primer CGSH2R1 (ATC CGG TCT AGA CTA TTG GAG AGC AAC ACC ATC, SEQ ID NO: 46), to which a XbaI digestion site was added, under the above condition, and the amplification product was purified by using QIAquick PCR purification Kit. The purified PCR product and the pAUR123 vector (Takara Shuzo Co.) were digested with restriction enzymes KpnI and XbaI, then ligated to each other and used to transform Escherichia coli JM109 competent cells. Thus, the CGSH2/pAUR123 vector was produced. Subsequently, the CGSH2/pAUR123 vector and the pYES2 vector were digested with a restriction enzyme BamHI. A region containing ORF of CGSH2 and a promoter derived from the pAUR123 vector was prepared from the CGSH2/pAUR123 vector, ligated to the pYES2 vector of which digested ends were dephosphorylated, and used to transform Escherichia coli JM109 competent cells. Thus, the CGSH2/pYES2 vector, an expression vector of the glutathione synthetase homologue of Candida utilis , was produced ( FIG. 3 ).
<5> Construction of Expression Vector of γ-Glutamylcysteine Synthetase Homologue Derived from Candida utilis
PCR was performed by using chromosomal DNA of Candida utilis as a template to amplify the ORF region of the γ-glutamylcysteine synthetase gene homologue (CGSH1) of Candida utilis . PCR was performed by using Pyrobest (Takara Shuzo Co.) according to the manufacturer's instruction under the conditions of a reaction at 98° C. for 2 minutes, followed by 40 cycles of reactions at 98° C. for 20 seconds, 55° C. for 30 seconds and 72° C. for 2 minutes. PCR was performed by using a N-terminus primer CGSH1F1 (GAG TAC GGT ACC ATG GGG CTG CTA TCA TTA GGG AC, SEQ ID NO: 47), to which a KpnI digestion site was added, and a C-terminus primer CGSH1R1 (CCC TTA TCT AGA TTA AGC CTT TGG GTT GTT TAT C, SEQ ID NO: 48), to which a XbaI digestion site was added, under the above condition, and the amplification product was purified by using QIAquick PCR purification Kit. The purified PCR product and the pAUR123 vector (Takara Shuzo Co.) were digested with restriction enzymes KpnI and XbaI, then ligated to each other and used to transform Escherichia coli JM109 competent cells. Thus, the CGSH1/pAUR123 vector was produced. Subsequently, the CGSH1/pAUR123 vector and the pYES2 vector were digested with a restriction enzyme BamHI. A region containing ORF of CGSH1 and a promoter derived from the pAUR123 vector was prepared from the CGSH1/pAUR123 vector, ligated to the pYES2 vector of which digested ends were dephosphorylated, and used to transform Escherichia coli JM109 competent cells. Thus, the CGSH1/pYES2 vector, an expression vector of the γ-glutamylcysteine synthetase homologue of Candida utilis , was produced ( FIG. 4 ).
<6> Complementation Test
Subsequently, transformants were obtained by introducing the CGSH1/pYES2 vector or the CGSH2/pYES2 vector into the Nα3 strain and the Nα4 strain. The Nα3 strain or the Nα4 strain was precultivated and inoculated in 50 ml of a liquid medium (YPD medium containing 1 mM glutathione) until the cells reached the logarithmic growth phase. The cultivated cells were suspended in 1 M sorbitol, mixed with the CGSH1/pYES2 vector or the CGSH2/pYES2 vector and transformed by electroporation. The transformed strains were cultivated on an SD plate containing 1 mM glutathione, and grown strains were selected. The Nα3 strain and the Nα4 strain showed uracil auxotrophy and could grow in the SD medium only when the CGSH1/pYES2 vector or the CGSH2/pYES2 vector was contained. Using the conventional manner, the obtained transformants was confirmed to contain the CGSH1/pYES2 vector or the CGSH2/pYES2 vector.
Thus, transformant Nα3/CGSH1 strain, Nα4/CGSH1 strain, Nα3/CGSH2 strain and Nα4/CGSH2 strain were obtained. Nα3/CGSH1 showed sensitivity to 2 mM methylglyoxal, whereas the Nα4/CGSH1 strain did not show sensitivity to 2 mM methylglyoxal. On the other hand, Nα3/CGSH2 strain showed no sensitivity to 2 mM methylglyoxal, whereas the Nα4/CGSH2 strain showed sensitivity to 2 mM methylglyoxal. Further, when cultivated in the SD medium, the Nα3/CGSH1 strain and Nα4/CGSH2 strain contained almost no glutathione in the logarithmic growth phase, whereas the Nα4/CGSH1 strain and Nα3/CGSH2 strain contained 0.4% glutathione. The Nα3 strain had reduced glutathione synthesis ability due to a mutation occurred in glutathione synthetase of Saccharomyces cerevisiae , whereas the Nα4 strain had a reduced glutathione synthesis ability due a mutation occurred in γ-glutamylcysteine synthetase of Saccharomyces cerevisiae . Thus, it was demonstrated that CGSH1 complemented γ-glutamylcysteine synthetase of Saccharomyces cerevisiae and that CGSH2 complemented glutathione synthetase of Saccharomyces cerevisiae.
Examples of producing Candida utilis in which glutathione synthetase-activity has been reduced by means of a part of DNA encoding glutathione synthetase are shown below as Reference Examples. Candida utilis in which glutathione synthetase activity has been reduced can also be produced in a similar method by means of a DNA which comprises the nucleotide sequence of the nucleotide numbers from 58 to 981 of SEQ ID NO: 1, or a DNA which is hybridizable with the DNA comprising the sequence of the nucleotide numbers from 58 to 981 of SEQ ID NO: 1 or a probe that can be prepared from the nucleotide sequence under stringent conditions.
Reference Example 1
Production of Gene Disruption Cassette for Glutathione Synthetase of Candida utilis
PCR was performed by using chromosomal DNA prepared from Candida utilis ATCC15239 as a template and the primers of SEQ ID NOS: 49 and 50. PCR was performed by using Pyrobest (Takara Shuzo Co.) according to the manufacturer's instruction under the conditions of a reaction at 98° C. for 2 minutes, followed by 40 cycles of reactions at 98° C. for 20 seconds, 55° C. for 30 seconds and 72° C. for 2 minutes.
The amplified product was purified by using QIA Quick PCR Purification Kit (QIAGEN Co., Cat. No. 28106), and the purified product was added with adenine at the termini and ligated to the pGEM-T Easy vector. The addition of adenine was performed by using AmpliTaq (ABI, Code N 8 O 8 -0161) and 2.5 mM DATP instead of dNTP by a reaction at 72° C. for 10 minutes. Subsequently, nucleotide sequences at two sites in the cloned fragment were replaced by site-directed recombination using QuikChange Site-Directed Mutagenesis Kit (STRATAGENE Co., Catalog #200518) to introduce HindIII and KpnI digestion sites and thereby obtain a CGSH2Ctermi/pGEMT-Easy vector. The primers used for the site-directed recombination are shown below.
[First Introduction of Mutation (Introduction of HindIII Digestion Site)]
GAA GCC TCA GCA TGA AGC TTG TGG (SEQ ID NO: 51) TAA TAA CAT TTA C G TAA ATG TTA TTA CCA CAA GCT TCA (SEQ ID NO: 52) TGC TGA GGC TTC
[Second Introduction of Mutation (Introduction of KpnI Digestion Site)]








CGA CCA ATC GAC TGG TAC CGT TAT
(SEQ ID NO: 53)


CAA AAA CTC TG



CA GAG TTT TTG ATA ACG GTA CCA GTC
(SEQ ID NO: 54)

GAT TGG TCG












Further, PCR was performed by using chromosomal DNA prepared from Candida utilis ATCC15239 as a template and the following primers to amplify a fragment containing the URA3 gene. PCR was performed by using KOD Dash (TOYOBO Co., Code LDP-101) according to the manufacturer's instruction under the conditions of a reaction at 94° C. for 2 minutes, followed by 30 cycles of reactions at 94° C. for 1 minute, 54° C. for 30 seconds and 74° C. for 40 seconds.







CCC AAG CTT CTC TAC TTG CTT CTG
(SEQ ID NO: 55)


CTC AAC



GCA GGT ACC AAC TTC CGA AAA CAG
(SEQ ID NO: 56)

TAA TGA AC












The amplification product was ligated to the pGEMT-Easy vector to obtain a CURA3/pGEMT-Easy vector.
The CGSH2Ctermi/pGEMT-Easy vector and the CURA3/pGEMT-Easy vector were each digested with HindIII and KpnI. A fragment containing CGSH2 and a main part of pGEMT-Easy was prepared from the CGSH2Ctermi/pGEMT-Easy vector, and a fragment containing CURA3 was prepared from the CURA3/pGEMT-Easy vector. These prepared fragments were ligated to each other and used to transform Escherichia coli JM109. Thus, a CURA3ACGSH2/pGEMT-Easy vector was produced.
PCR amplification was performed by using the CURA3ACGSH2/pGEMT-Easy vector digested with a restriction enzyme NotI as a template and the primers of SEQ ID NOS: 49 and 50. Thus, a gene disruption cassette for glutathione synthetase of Candida utilis was produced ( FIGS. 5 and 6 ).
Reference Example 2
Acquisition of Uracil Auxotrophic
Candida utilis ATCC15239ura-strain
ATCC15239ura-strain, a uracil-auxotrophic strain derived from ATCC15239, was obtained in a conventional manner (the technique of Luis et al., refer to FEMS Microbiology Letters 165, 335-340 (1998)). Since the ATCC15239ura-strain was complemented by the URA3 gene as described later, this strain is expected to be a ura3 mutant.
Reference Example 3
Acquisition of γ-glutamylcysteine Producing Yeast (ATCC15239Δgsh2 strain) Derived from Candida utilis
First, the ATCC15239ura-strain was cultivated overnight at 30° C. in a YPD test tube medium. The broth culture was inoculated in a YPD flask medium (500 ml Sakaguchi flask, 50 ml filled) and cultivated at 30° C. with shaking. The cells were collected in the logarithmic growth phase and washed three times with a 1 M sorbitol solution cooled to 4° C. The washed cells were suspended in a cooled 1 M sorbitol solution. The suspension was added with 50 μl (2 μg) of the glutathione synthetase gene disruption cassette, mixed well in a 0.2 cm cuvette and subjected to electroporation by using Gene Pulser System (BioRad Co.) with impedance of 200 Ω, capacitance of 125 μF and set voltage of 1.5 kV. 1 ml of cooled 1 M sorbitol was poured into the cuvette, and the cuvette was cooled on ice for 10 minutes. The cell suspension was spread over an SD plate and cultivated at 30° C.
The strains grown on the plate were replicated on an SD plate and an SD plate containing 10 mM methylglyoxal, and cultivated at 30° C. 7 strains showing sensitivity to methylglyoxal were selected. These 7 strains were each cultivated overnight at 30° C. in a YPD test tube medium with shaking. The culture broth was inoculated in an amount of 2% to an SD medium (500 ml Sakaguchi flask, 50 ml filled) and cultivated at 30° C. with shaking. The cells in the logarithmic growth phase were collected and washed twice with sterilized water. The washed cells were extracted with hot water at 70° C. for 10 minutes, and γ-glutamylcysteine extracted from the yeast cells was isolated and quantified by HPLC. Further, after placing the washed yeast cells contained in a certain amount of medium on filter paper and heating at 105° C. for 4 hours, the weight of the remaining cells was measured as the dry cell weight of the yeast. Thus, the ATCC15239Δgsh2 strain was obtained as a Candida utilis strain containing 1% or more of γ-glutamylcysteine based on dry yeast cells.
The ATCC15239Δgsh2 strain was inoculated in an SD medium and cultivated at 30° C. for 2 days with shaking. The culture was inoculated at a concentration of 2% in an SD medium and cultivated at 30° C. with shaking. The γ-glutamylcysteine contents measured were 1.08% and 1.12% after 7 hours and 9 hours, respectively. Glutathione content was below the detectable limit.
Reference Example 4
Measurement of Glutathione Synthetase Activity of ATCC15239Δgsh2 Strain
The ATCC 15239Δgsh2 strain was inoculated in a YPD medium and cultivated at 30° C. with shaking. The culture was inoculated at a concentration of 2% in an SD medium (2-L finned conical flask, filled 400 ml) and cultivated at 30° C. with shaking. Cells in the logarithmic growth phase were collected and washed twice with 1 M sorbitol cooled to 4° C. The washed cells were suspended in 0.5 ml of 10 mM Tris-HCl buffer (pH 7.5) containing 0.1 mM phenylmethanesulfonyl fluoride (PMSF). The suspension was added with glass beads (GLASS BEADS 425-600 Microns Acid-Washed (SIGMA Co., Code G-8772)), and the cells were disrupted by using BEAD-BEATER (WAKENYAKU Co.). The disruption of cells was confirmed microscopically. Then, 1 ml of the aforementioned buffer was added, and the glass beads and cell debris were removed by centrifugation. Thus, a crude cell extract was obtained. The crude cell extract was purified by using ULTRAFREE-15 Biomax 10 (MILLIPORE Co., Cat.No.UFV2BGC40) to obtain an enzyme solution. The protein content in the obtained enzyme solution was quantified by the Bradford method. Color development was attained by using Protein Assay CBB Solution (Nakarai Co., Code 29449-15), and absorbance at 595 nm was measured. A standard curve was created by using Albumin Standard (PIERCE Co., No. 23210).
The glutathione synthetase activity in the enzyme solution obtained as described above was measured according to the method of Gushima et al. (T. Gushima et al., J. Appl. Biochem., 5, 210 (1983)) as follows.
[Reaction Mixture]












100 mM γ-glutamylcysteine
100
μl


100 mM MgCl 2
100
μl


50 mM ATP
100
μl


100 mM Gly
100
μl


1 M Tris-HCl (pH 8.0)
85.5
μl


160 mM PEP
12.5
μl


1 mg/ml PK
2
μl


Enzyme solution (1 to 10 mg protein)


Purified water


Total
2
ml





PEP: phosphoenolpyruvic acid (SIGMA Co., Code P-7127)


PK: pyruvate kinase (SIGMA Co., Code P-1903)























The reaction mixture having the above composition was allowed to react at 30° C. for 0 to 2 hours in the presence of the enzyme in an amount of 1 to 10 mg protein. The reaction mixture was added with 1/5 equivalent of methacrylic acid to terminate the reaction and then adjusted to pH 8.0, and the amount of the produced GSH was determined. The enzymatic activity at this time was below the detectable limit and was not detected.
Subsequently, the glutathione synthetase activity of the ATCC15239 strain, which is a parent strain of the ATCC15239Δgsh2 strain, was similarly measured. As a result, the glutathione synthetase activity of the ATCC15239 strain was 0.383 μmol-GSH/mg protein/hour. Thereby, a Candida utilis which has been modified to have glutathione synthetase activity of 0.003 mmol GSH/mg protein/hour or less was obtained by means of the DNA of the present invention.
INDUSTRIAL APPLICABILITY
The present invention provides glutathione synthetase gene and γ-glutamylcysteine synthetase gene of Candida utilis . Yeast extracts that can be used for improving flavor and taste of food and so forth can be produced at a low cost by using the Candida utilis bred by means of the genes of the present invention.
While the invention has been described in detail with reference to preferred embodiments thereof, it will be apparent to one skilled in the art that various changes can be made, and equivalents employed, without departing from the scope of the invention. Each of the aforementioned documents, including the foreign priority document, JP 2003-310084, is incorporated by reference herein in its entirety.