Regulation of pyc1 encoding pyruvate carboxylase isozyme I by nitrogen sources in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, the existence of PYC1 and PYC2 encoding cytoso lic pyruvate carboxylase isoform I and II is rather puzzling, owing to the lack of potent differential gene regulation by the carbon sources. We repor t several findings indicating that these two genes are differentially regul ated by the nature of the nitrogen source. In wild-type cells, the activity of pyruvate carboxylase, which is the sum of pyruvate carboxylase isoform I and II, was two- to fivefold lower in carbon medium containing aspartate, asparagine, glutamate or glutamine instead of ammonium as the nitrogen sou rce, whereas it was 1.5- to threefold higher when the ammonium source was s ubstituted by arginine, methionine, threonine or leucine. These enzymatic c hanges were independent of the nature of the carbon source and closely corr elated to the changes in beta -galactosidase from PYC1-lacZ gene fusion and in PYC1 transcripts. Transfer of exponentially growing cells of the pyc2 m utant from an aspartate or a glutamate medium to an ammonium medium caused a fivefold increase in PYC1 mRNA in less than 30 min, whereas in the invers e experiment, PYC1 transcripts returned within 30 min to the low levels fou nd in aspartate/glutamate medium. By contrast, these conditions affected ne ither the pyruvate carboxylase activity encoded by PYC2 nor PYC2 mRNA. Cons idering that changes in PYC1 expression inversely correlated with changes i n alpha -ketoglutarate concentration or in alpha -ketoglutarate/glutamate r atio following the nitrogen shift experiments, and taking into account the pivotal role of this metabolite in ammonium assimilation, it is suggested t hat changes in alpha -ketoglutarate or in the alpha -ketoglutarate/glutamat e ratio might be implicated in triggering the nitrogen effects on PYC1 expr ession. The physiological significance of the differential sensitivity of P YC1 and PYC2 genes with respect to the nitrogen source in the growth medium is also discussed.