Autoregulation may control the expression of yeast pyruvate decarboxylase structural genes PDC1 and PDC5.

Recently we deleted the pyruvate decarboxylase structural gene PDC1 from the genome of the yeast Saccharomyces cerevisiae. The pdc1 deletion mutants had pyruvate decarboxylase activity due to the presence of a second structural gene [Schaaff, I., Green, J. B. A., Gozalbo, D. & Hohmann, S. (1989) Curr. Genet. 15, 75-81]. We cloned and sequenced this gene which we call PDC5. The predicted amino acid sequences of PDC1 and PDC5 are 88% identical. Deletion of PDC5 did not cause any decrease in the specific pyruvate decarboxylase activity while pdc1 deletion mutants had 80% of the wild-type activity. Deletion mutants lacking both PDC1 and PDC5 did not show any detectable pyruvate decarboxylase activity in vitro and were unable to ferment glucose. This indicates that PDC1 and PDC5 are the only structural genes for pyruvate decarboxylase in yeast. The PDC5 isoenzyme showed a slightly higher Km value for its substrate pyruvate than the PDC1 product (PDC5: Km = 8 mM; PDC1: Km = 5 mM), as measured in crude extract of pdc1 and pdc5 deletion mutants, respectively. PDC5 is only expressed in pdc1 deletion mutants. No mRNA transcribed from PDC5 could be detected in wild-type cells. Thus, in addition to the control by glucose induction, pyruvate decarboxylase activity seems to be subject to autoregulation. Similar phenomena have been described previously for tubulin, histones and a ribosomal protein but not for metabolic enzymes.