Transcriptional regulation of most phospholipid biosynthetic genes in Saccharomyces cerevisiae is coordinated by inositol and choline. Inositol affects phosphatidic acid (PA) intracellular levels. Opi1p interacts physically with PA and is the main repressor of the phospholipid biosynthetic genes. It is localized in the endoplasmic reticulum (ER) bound to the ER membrane protein Scs2p. When PA levels drop, Opi1p is translocated into the nucleus repressing most phospholipid biosynthetic genes. The OPI1 locus was identified in a screen looking for overproduction and excretion of inositol (Opi-). Opi- mutants are generally associated with a defect in repression of the phospholipid biosynthetic genes. Using a conditional shut-off library we conducted a screen that identified 121 genes with an Opi- phenotype. These genes identified pathways previously unknown to regulate the phospholipid genes like the Ubiquitin/Proteasome system. It also identified the essential subunits of NuA4 HAT. Genes involved in the Ubiquitin/Proteasome system and NuA4 HAT were tested for a repression defect in the most highly regulated phospholipid biosynthetic genes, INO1. Neither mutant identified from these pathways showed a repression defect under repressing conditions. Phospholipid biosynthetic genes are also growth phase regulated that is under activating conditions (no inositol) INO1 is active, but when cells reach the stationary phase INO1 is repressed. Both NuA4 HAT and Ubiquitin/Proteasome genes showed a repression defect at the stationary phase of the cellular growth suggesting that these biological processes are responsible for the regulation of INO1 at the stationary phase of the cellular growth
