REGULATION OF THE PDA1 GENE ENCODING THE E1-ALPHA-SUBUNIT OF THE PYRUVATE-DEHYDROGENASE COMPLEX FROM SACCHAROMYCES-CEREVISIAE

Expression of the PDA1 gene encoding the E1alpha subunit of the pyruva te dehydrogenase complex (PDH complex) and activity of the complex wer e investigated in cells grown under several conditions. Comparable amo unts of PDA1 mRNA and E1alpha subunit were detected in cells from batc h and chemostat cultures grown on various carbon sources, showing cons titutive expression of PDA1 at the transcriptional and translational l evels. Induction of the regulatory GCN4 mechanism upon histidine starv ation, using the anti-metabolite 3-amino-1,2,4-triazole, increased the levels of PDA1 mRNA by approximately 40%. However, a corresponding in crease of E1alpha concentration or activity of the PDH complex could n ot be detected. Hence, expression of the PDA1 gene is only regulated t o a small extent, if at all, by the GCN4 mechanism. Contrary to the co nstant levels of PDA1 mRNA and E1alpha subunit in both batch and chemo stat cultures, the specific activity of the PDH complex varied with th e culture conditions. The activity of the PDH complex in chemostat cul tures was approximately two-threefold higher than in batch cultures gr own on the same carbon sources. Overproduction of the E1alpha subunit in batch cultures resulted in a two-threefold increase in the activity of the PDH complex. Taken together, these results indicate that the a ctivity of the PDH complex is mainly regulated by post-translational m odification of the E1alpha subunit. Expression of PDA1 and activity of the PDH complex were also detected in cultures grown under conditions where no physiological significance of the PDH complex was expected, i.e. during anaerobic growth on glucose or aerobic growth on ethanol. Apparently, the switch from oxidative growth to fermentation occurs wi thout much effect on the PDH complex. These observations suggest that the PDH complex has an alternative function besides sugar catabolism.