Expression of the PDA1 gene encoding the E1α subunit of the pyruvate dehydrogenase complex (PDH complex) and activity of the complex were investigated in cells grown under several conditions. Comparable amounts of PDA1 mRNA and E1α subunit were detected in cells from batch and chemostat cultures grown on various carbon sources, showing constitutive expression of PDA1 at the transcriptional and translational levels. Induction of the regulatory GCN4 mechanism upon histidine starvation, using the anti-metabolite 3-amino-1,2,4-triazole, increased the levels of PDA1 mRNA by approximately 40%. However, a corresponding increase of E1α concentration or activity of the PDH complex could not be detected. Hence, expression of the PDA1 gene is only regulated to a small extent, if at all, by the GCN4 mechanism.
Contrary to the constant levels of PDA1 mRNA and E1α subunit in both batch and chemostat cultures, the specific activity of the PDH complex varied with the culture conditions. The activity of the PDH complex in chemostat cultures was approximately two–threefold higher than in batch cultures grown on the same carbon sources. Overproduction of the E1α subunit in batch cultures resulted in a two–threefold increase in the activity of the PDH complex. Taken together, these results indicate that the activity of the PDH complex is mainly regulated by post-translational modification of the E1α 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 without much effect on the PDH complex. These observations suggest that the PDH complex has an alternative function besides sugar catabolism.