The budding yeast Saccharomyces cerevisiae changes its gene expression profile when environmental nutritional conditions are changed. Protein kinases including cyclic AMP-dependent kinase, Snf1 and Tor kinases play important roles in this process. Pho85 kinase, a member of the yeast cyclin-dependent kinase family, is involved in the regulation of phosphate metabolism and reserve carbohydrates, and thus is implicated to function as a nutrient-sensing kinase. Upon depletion of glucose in the medium, yeast cells undergo a diauxic shift, accompanied by a carbon metabolic pathway shift, stimulation of mitochondrial function and downregulation of ribosome biogenesis and protein synthesis. We analysed the effect of a pho85Δ mutation on the expression profiles of the genes in this process to investigate whether Pho85 kinase participates in the yeast diauxy. We found that, in the absence of PHO85, a majority of mitochondrial genes were not properly induced, that proteasome-related and chaperonin genes were more repressed, and that, when glucose was still present in the medium, a certain class of genes involved in ribosome biogenesis (ribosomal protein and rRNA processing genes) was repressed, whereas those involved in gluconeogenesis and the glyoxylate cycle were induced. We also found that PHO85 is required for proper expression of several metal sensor genes and their regulatory genes. These results suggest that Pho85 is required for proper onset of changes in expression profiles of genes responsible for the diauxic shift.
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