CARBON SOURCE REGULATION OF PIS1 GENE-EXPRESSION IN SACCHAROMYCES-CEREVISIAE INVOLVES THE MCM1 GENE AND THE 2-COMPONENT REGULATORY GENE, SLN1

The Saccharomyces cerevisiae PISI gene encodes phosphatidylinositol sy nthase. The amount of phosphatidylinositol synthase is not affected by the presence of inositol and choline in the growth medium. This is un usual because the amounts and/or activities of other phospholipid bios ynthetic enzymes are affected by these precursors, and the promoter of the PIS1 gene contains a sequence resembling the regulatory element t hat coordinates the inositol-mediated regulation (UAS(INO)). We found that transcription of the PIS1 gene was insensitive to inositol and ch oline and did not require the putative UAS(INO) regulatory sequence or the cognate regulatory genes (INO2 and OPI1). The PIS1 promoter inclu des sequences (MCEs) that hind the Mcm1 protein. Because the Mcm1 prot ein interacts with both the Sln1 and the Gal11 regulatory proteins, we examined the effect of mutant alleles of the MCMI and SLN1 genes and carbon source on expression of the PISI. gene. We found that expressio n of the PIS1 gene was reduced when cells were grown in a medium conta ining glycerol and increased when grown in a medium containing galacto se relative to cells grown in a glucose medium. The glycerol-mediated repression of PIS1 gene expression required both the MCM1 gene and the MCEs, whereas the SLN1 gene was required for full galactose-mediated induction of a PIS1-lacZ reporter gene. Thus, PISI gene expression is unique among the phospholipid biosynthetic structural genes because it is uncoupled from the inositol response and regulated in response to the carbon source. This is the first example in yeast of a complete ci rcuit linking a stimulus (carbon source) to gene regulation (PIS1) usi ng a two-component regulator (SLN1).