Expression of the Saccharomyces cerevisiae PIS1 gene is modulated by multiple ATGs in the promoter

Regulated transcription of the Saccharomyces cerevisiae phosphatidylinositol biosynthetic gene, PIS1, yields pleiotropic effects on phospholipid synthesis

Phosphatidylinositol is an important membrane lipid in Saccharomyces cerevisiae and other eukaryotes. Phosphatidylinositol and its metabolites (phosphoinositides, inositol polyphosphates, etc.) affect many cellular processes with implications in human diseases. Phosphatidylinositol synthesis in S. cerevisiae requires the essential PIS1 gene. Recent studies reveal that PIS1 expression is regulated at the level of transcription in response to carbon source, oxygen, and zinc. However, the consequence of this regulation on phosphatidylinositol levels and functions has not been thoroughly studied. To investigate this, we created a strain with a galactose-inducible GAL1-PIS1 gene. In this strain, the amount of phosphatidylinositol correlated with PIS1 expression but did not exceed c. 25% of the total phospholipid composition. Interestingly, we found that 4% phosphatidylinositol was sufficient for cell growth. We also found that reduced PIS1 expression yielded derepression of two phospholipid biosynthetic genes (INO1 and CHO1) and the INO2 regulatory gene. Consistent with this derepression, reduced PIS1 expression also yielded an overproduction of inositol (Opi(-)) phenotype. The effect on transcription of the INO1, CHO1, and INO2 genes is consistent with the accepted model that phosphatidic acid (PA) is the signal for regulation of these genes because decreased phosphatidylinositol synthesis would affect PA levels.

Transcription regulation of the Saccharomyces cerevisiae PIS1 gene by inositol and the pleiotropic regulator, Ume6p

In Saccharomyces cerevisiae, transcription of most of the phospholipid biosynthetic genes (e.g. INO1, CHO1, CHO2 and OPI3) is repressed by growth in the presence of inositol and choline and derepressed in their absence. This regulation requires the Ino2p and Ino4p activators and the Opi1p repressor. The PIS1 structural gene is required for the synthesis of the essential lipid phosphatidylinositol. Previous reports show that PIS1 expression is uncoupled from inositol/choline regulation, but is regulated by carbon source, hypoxia and zinc. However, in this study we found that the expression of PIS1 is induced twofold by inositol. This regulation did not require Ino2p and Ino4p, although Ino4p was required for full expression. Ino4p is a basic helix-loop-helix protein that requires a binding partner. Curiously, none of the other basic helix-loop-helix proteins affected PIS1 expression. Inositol induction did require another general regulator of phospholipid biosynthesis, Ume6p. Ume6p was found to be a positive regulator of PIS1 gene expression. Ume6p, and several associated factors, were required for inositol-mediated induction and chromatin immunoprecipitation analysis showed that Ume6p directly regulates PIS1 expression. Thus, we demonstrate novel regulation of the PIS1 gene by Ume6p.