THE 2 ACETYL-COENZYME-A SYNTHETASES OF SACCHAROMYCES-CEREVISIAE DIFFER WITH RESPECT TO KINETIC-PROPERTIES AND TRANSCRIPTIONAL REGULATION

Saccharomyces cerevisiae contains two structural genes, ACS1 and ACS2, each encoding an active acetyl-coenzyme A synthetase. Characterizatio n of enzyme activities in cell-free extracts from strains expressing e ither of the two genes revealed differences in the catalytic propertie s of the two enzymes, The K-m for ac etate of Acs1p was about 30-fold lower than that of Acs2p and Acs1p, but not Acs2p, could use propionat e as a substrate, Enzyme activity measurements and mRNA analyses showe d that ACS1 and ACS2 were both expressed during carbon-limited growth on glucose, ethanol, and acetate in aerobic chemostat cultures, In ana erobic glucose-limited cultures, only the ACS2 gene was expressed, Bas ed on these facts, the products of the ACS1 and ACS2 genes were identi fied as the previously described ''aerobic'' and ''non-aerobic'' forms of acetyl-coenzyme A synthetase, respectively, Batch and glucose-puls e experiments revealed that transcription of ACS1 is subject to glucos e repression, A mutant strain lacking Acs2p was unable to grow on gluc ose in batch cultures, but grew readily in aerobic glucose limited che mostat cultures, in which the low residual glucose concentration allev iated glucose repression. Experiments in which ethanol was pulsed to a erobic ethanol-limited chemostat cultures indicated that, in addition to glucose, ethanol also repressed ACS1 transcription, although to a l esser extent, In contrast, transcription of ACS2 was slightly induced by ethanol and glucose, Absence of ACS2 prevented complete glucose rep ression of ACS1, indicating that ACS2 (in)directly is involved in the transcriptional regulation of ACS1.