PDC10 MUTANTS OF SACCHAROMYCES-CEREVISIAE GIVE EVIDENCE FOR AN ADDITIONAL STRUCTURAL PDC GENE - CLONING OF PDC5, A GENE HOMOLOGOUS TO PDC1

The PDC1 gene coding for a pyruvate decarboxylase (PDC; EC 4.1.1.1) was deleted from the Saccharomyces cerevisiae genome. The resulting pdc1(0) mutants were able to grow on glucose and still contained 60 to 70% of the wild-type PDC activity. Two DNA fragments with sequences homologous to that of the PDC1 gene were cloned from the yeast genome. One of the cloned genes (PDC5) was expressed at high rates predominantly in pdc1(0) strains and probably encodes the remaining PDC activity in these strains. Expression from the PDC1 promoter in PDC1 wild-type and pdc1(0) strains was examined by the use of two reporter genes. Deletion of PDC1 led to increased expression of the two reporter genes regardless of whether the fusions were integrated into the genome or present on autonomously replicating plasmids. The results suggested that this effect was due to feedback regulation of the PDC1 promoter-driven expression in S. cerevisiae pdc1(0) strains. The yeast PDC1 gene was expressed in Escherichia coli, leading to an active PDC. This result shows that the PDC1-encoded subunit alone can form an active tetramer without yeast-specific processing steps.