Pathways of GABA assimilation and effect on Saccharomyces cerevisiae metabolism during wine fermentation.
Benoît Bach, Carole Camarasa, Sylvie Dequin
UMR SPO-Microbiologie, INRA, 2 place Pierre Viala, Montpellier, 34060, France
Genome wide expression analyses during wine fermentation have recently shown that the genes of the GAD-GABA (gamma-aminobutyrate) pathway ( GAD1, UGA1 and UGA2 ) are up-regulated during the nitrogen-limited stationary phase [Rossignol et al., 2003. Yeast 20, 1369-85]. To investigate potential roles of this pathway in glutamate catabolism and GABA assimilation, functional analyses were carried out in the wine-derived strain V5 under wine fermentation conditions. Expression studies and analysis of the impact of GAD1 and UGA2 deletion show that glutamate can be catabolized through this pathway although at a very low rate, without significant impact on the amount of succinate produced. On the other hand, external GABA can be efficiently converted to succinate via Uga2p and Uga1p, with a yield up to 0.8 mol/mol. The assimilation of GABA causes alteration of yeast metabolism, as the result of increased demand for alpha-ketoglutarate and of the necessity to balance the excess of NAD(P)H generated through the Uga2p reaction. Interestingly, we show that a significant part of GABA can be consumed without being converted in succinate. In addition, the uga2 mutant can use GABA without accumulation of the intermediate succinate semi-aldehyde (SSA). These data strongly suggest the existence in yeast of a novel pathway from SSA which might be activated in the presence of GABA. The characterisation of this pathway is in progress.