Suppression of pdc2 regulating pyruvate decarboxylase synthesis in yeast

Mutants lacking pyruvate decarboxylase cannot grow on glucose. We have isolated three different complementation groups of extragenic suppressors that suppress mutations in pdc2, a regulatory locus required for the synthesis of the glycolytic enzyme pyruvate decarboxylase. The most frequent of these is a recessive mutation in the structural gene PFK1 of the soluble phosphofructokinase. The other class XSP18 (extragenic suppressor of pdc2) is a dominant temperature-sensitive suppressor that allows the cells to grow on glucose only at 30 degrees but not at 36 degrees. It also affects the normal induction of the glucose-inducible enolase 2, which can be rescued by providing a copy of wild-type xsp18 in trans-heterozygotes. The pyruvate decarboxylase activity in the triple mutant pdc2 pfk1 XSP18 is nearly equal to the sum of the activities in the two double mutants pdc2 pfk1 and pdc2 XSP18, respectively. This implies that the two suppressors act through independent pathways or that there is no cooperativity between them. In the pdc2 pfk1 XSP18, strain, pfk1 suppresses the loss of induction of glucose-inducible enolase 2 brought about by XSP18 but fails to rescue temperature sensitivity. The third class (xsp37) supports the growth of the pdc2 mutant on glucose but fails to support growth on gluconeogenic carbon sources. All the three suppressors suppress pdc2 delta as well and act on PDC1 at the level of transcription.