清酒酵母の胞子形成に関する研究
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概要
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Defective sporulation of industrially used sake yeasts(Saccharomyces cerevisiae)such as Kyokai no. 7 and no. 9 is one of the most troublesome problems inhibiting the breeding-by hybridization and/or genetic analysis-of yeast strains with improved characteristics for sake making. In order to clarify the cause of defective sporulation in industrially used sake yeasts, and thereby facilitate their molecular breeding, the sporulation-defective properties of Kyokai no. 7 were analyzed in nutrition-controlled media and compared with a wild-type diploid yeast(strain 4011)as a control. An analysis of the sporulation process of strain 4011 in a nutrient-starved medium indicated that Ca^<2+>, glutathione(GSH)and some other biomolecules were able to enhance sporulation. Of these, Ca^<2+> and GSH were prerequisites for sporulation. The basic amino acid D-, L-lysine was found to induce sporulation in cells growing in a nutrient-rich medium. An analysis of the sporulation of strain 4011 and Kyokai no. 7 in a lysine-dependent system showed no significant increase in IME1 transcript and a decrease in cAMP levels throughout the sporulation, thus suggesting that at least a decreased level of cAMP is not required for entry into meiosis. Although the formation of 4-spored asci and spore viability were low, the addition of GSH, its analogues, and other biomolecules to the nutrition-starved medium apparently improved the sporulation of Kyokai no. 7. This result indicated that the defective sporulation of Kyokai no. 7 was mainly due to metabolic imbalance, and not to any serious genetic disorder upstream of the signal transduction pathway. The effects on sporulation of metabolic properties such as ethionine-resistance, inability to utilize β-alanine at 35℃, and loss of acid phosphatase activity, all of which are inherent characteristics of Kyokai no. 7,were thus investigated. The elimination of any one of these properties was found to restore the sporulation of Kyokai no. 7. For example, a mutation with ethionine-sensitivity showed a marked increase in the formation of 4-spored asci(80%)and in the viability of the spores(over 25%). These findings were applicable to other Kyokai strains apart from no. 7,and will be utilized as a general method for the molecular breeding and genetic analysis of industrially important brewery yeasts.
- 1996-03-25