Breeding sake yeast and identification of mutation patterns by synchrotron light irradiation

Breeding High-Yield Ethyl Caproate-Producing Saccharomyces cerevisiae in Sake: Flux Regulation from Glycolytic Fermentation to the FAS Pathway and Alcohol Acyltransferase Overexpression

Ethyl caproate is the characteristic aroma compound with an apple-like scent in Ginjoka sake. However, the medium-chain acyl-CoA flux of the fatty acid synthesis (FAS) pathway originating from glycolytic fermentation and the precursor-induced alcohol acyltransferase (AAT) activity by natural yeast limits the ethyl caproate content in sake. Here, we established combinatorial strategies involving genetic engineering and adaptive laboratory evolution (ALE) to increase the ethyl caproate production by Saccharomyces cerevisiae. In this study, we screened Saccharomyces cerevisiae YH-2, which exhibited high ethanol and ester yields , achieving a trade-off between FAS flux and energy metabolism. Subsequently, the cerulenin-resistant mutant strain YH-2-34, after 15 passages of adaptive domestication, produced 4.13 times more caproic acid than the wild type. This increase is attributed to the G1250S variation in the FAS2 sequences, which mediate acyl-CoA chain length in the FAS pathway, thereby producing more caproyl-CoA as the precursor. While AAT activity increased 2.40 times in the mutant YH-2-34, both EEB1 and EHT1 genes, which together encode AAT responsible for esterifying ethyl caproate, played critical roles. Although pEEB1s overexpression affected cell viability and ethyl caproate production, pEHT1s overexpression successfully increased the yield of ethyl caproate during post-fermentation. Finally, the yield of YH-2-34 with EHT1 overexpression achieved a significant increase from 1.21 to 7.40 mg/L in sake fermentation. By regulating the flux from glycolytic fermentation to the FAS pathway and overexpressing AAT, we constructed a high-yield ethyl-caproate-producing Saccharomyces cerevisiae strain. This may bring practical transformations to traditional brewing industries.

Development of sake yeast breeding and analysis of genes related to its various phenotypes

Sake is a traditional Japanese alcoholic beverage made from rice and water, fermented by the filamentous fungi Aspergillus oryzae and the yeast Saccharomyces cerevisiae. Yeast strains, also called sake yeasts, with high alcohol yield and the ability to produce desired flavor compounds in the sake, have been isolated from the environment for more than a century. Furthermore, numerous methods to breed sake yeasts without genetic modification have been developed. The objectives of breeding include increasing the efficiency of production, improving the aroma and taste, enhancing safety, imparting functional properties, and altering the appearance of sake. With the recent development of molecular biology, the suitable sake brewing characteristics in sake yeasts, and the causes of acquisition of additional phenotypes in bred yeasts have been elucidated genetically. This mini-review summarizes the history and lineage of sake yeasts, their genetic characteristics, the major breeding methods used, and molecular biological analysis of the acquired strains. The data in this review on the metabolic mechanisms of sake yeasts and their genetic profiles will enable the development of future strains with superior phenotypes.