Isolation and characterization of sake yeast mutants deficient in gamma-aminobutyric acid utilization in sake brewing

Sake yeasts take up gamma-aminobutyric acid (GABA) derived from rice-koji in the primary stage of sake brewing. The GABA content in sake brewed with the UGA1 disruptant, which lacked GABA transaminase, was higher than that brewed with the wild-type strain K701. The UGA1 disruptant derived from sake yeast could not grow on a medium with GABA as the sole nitrogen source. We have isolated the sake yeast mutants of K701 that were unable to grow on a medium containing GABA as the sole nitrogen source. The growth defect of GAB7-1 and GAB7-2 mutants on GABA plates was complemented by UGA1, which encodes GABA transaminase, and UGA2, which encodes succinic semialdehyde dehydrogenase (SSADH), respectively. DNA sequence analysis revealed that GAB7-1 had a homozygous nonsense mutation in UGA1 and GAB7-2 had a heterozygous mutation (G247D) in UGA2. The GABA transaminase activity of GAB7-1 and the SSADH activity of GAB7-2 were markedly lower than those of K701. These GAB mutants displayed a higher intracellular GABA content. The GABA contents in sake brewed with the mutants GAB7-1 and GAB7-2 were 2.0 and 2.1 times higher, respectively, than that brewed with the wild-type strain K701. These results suggest that the reduced function of the GABA utilization pathway increases the GABA content in sake.