Effect of the pho85 mutation on the catabolite repression of the CIT1 gene in yeasts Saccharomyces cerevisiae

The Krebs cycle is one of the major metabolic pathways in a cell, which includes both catabolic and anabolic reactions. The first enzyme of the Krebs cycle, citrate synthase, catalyzes one of a few irreversible reactions of the cycle, citrate formation from acetyl-CoA and oxaloacetate. Expression of the CIT1 gene encoding the mitochondrial form of this enzyme in Saccharomyces cerevisiae is repressed on glucose- and glutamate-containing medium and activated on the raffinose-containing medium. In this work, the dependence of glucose repression of the CIT1 gene on the content of phosphate in the medium was studied. On the phosphate-deficient medium, the level of the CIT1 gene expression was increased twice. A low-molecular-weight (about 34 kDa) protein was identified and shown to interact with a region of the CIT1 gene promoter (from -367 to -346 bp), which controls the glucose repression. The results obtained suggest that the Pho4 protein is involved in regulation of the CIT1 gene expression on the glucose-containing and phosphate-deficient medium. Disruption of the PHO85 gene encoding phosphoprotein kinase (Pho4p is the substrate of this enzyme) leads to alleviation of glucose repression of the CIT1 gene. Thus, in yeast cells grown in the presence of glucose, the PHO85 gene mediates downregulation of the CIT1 expression.