Abstract
The Saccharomyces cerevisiae PIS1 gene encodes phosphatidylinositol synthase. The amount of phosphatidylinositol synthase is not affected by the presence of inositol and choline in the growth medium. This is unusual because the amounts and/or activities of other phospholipid biosynthetic enzymes are affected by these precursors, and the promoter of the PIS1 gene contains a sequence resembling the regulatory element that coordinates the inositol-mediated regulation (UASINO). We found that transcription of the PIS1 gene was insensitive to inositol and choline and did not require the putative UASINO regulatory sequence or the cognate regulatory genes (INO2 and OPI1). The PIS1 promoter includes sequences (MCEs) that bind the Mcm1 protein. Because the Mcm1 protein interacts with both the Sln1 and the Gal11 regulatory proteins, we examined the effect of mutant alleles of the MCM1 and SLN1 genes and carbon source on expression of the PIS1 gene. We found that expression of the PIS1 gene was reduced when cells were grown in a medium containing glycerol and increased when grown in a medium containing galactose 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, PIS1 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 circuit linking a stimulus (carbon source) to gene regulation (PIS1) using a two-component regulator (SLN1).
Highlights
The PIS1 promoter includes sequences (MCEs) that bind the Mcm1 protein
PIS1 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
The UASINO Element in the PIS1 Promoter Is Inactive—The presence of a UASINO-like element in the PIS1 promoter (Fig. 1B) suggested that PIS1 transcription might be regulated by the cognate regulatory genes, INO2 and OPI1 (8 –10)
Summary
The PIS1 promoter includes sequences (MCEs) that bind the Mcm protein. Because the Mcm protein interacts with both the Sln and the Gal regulatory proteins, we examined the effect of mutant alleles of the MCM1 and SLN1 genes and carbon source on expression of the PIS1 gene. PIS1 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. The two-component regulatory systems include a sensor and a response regulator that modulate gene expression in reaction to changes in the environment [1]. Years, they have only recently been identified in Saccharomyces cerevisiae [2,3,4,5] An example of this is the yeast SLN1 gene product that includes both a sensor and a response regulator [3]. Two questions that need to be addressed are which environmental cues are sensed by this two-component regulatory system, and which genes are targeted for a response. We report that regulation of PIS1 gene expression by carbon sources involves the SLN1 and MCM1 genes
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