The Saccharomyces cerevisiae INO4 gene encodes a small, highly basic protein required for derepression of phospholipid biosynthetic enzymes.

The INO4 gene product is believed to be a positive regulatory factor in a complex cascade of positive and negative factors that coordinates the synthesis of phospholipids in the yeast Saccharomyces cerevisiae. Mutations at the INO4 locus result in a decrease in phosphatidylcholine synthesis and an inability to derepress the structural genes for inositol-1-phosphate synthase and phosphatidylserine synthase. In the present study, the transcript encoding the INO4 gene product has been identified and a transcription map of the INO4 region has been constructed. An ino4 deletion mutant was constructed by in vitro gene disruption and the deletion mutant was shown to be viable but auxotrophic for inositol. The deletion mutant expressed repressed levels of inositol-1-phosphate synthase (INO1) mRNA and exhibited reduced phosphatidylcholine biosynthesis, a phenotype similar to previously characterized ino4 mutants. The INO4 gene has been mapped to chromosome 15 and is tightly linked to the SUF1 tRNA gene. Translation of the DNA sequence of the INO4 gene results in a very basic protein of molecular weight 17,378. Computer analysis of the INO4 protein sequence identified several potential phosphorylation sites as well as several regions that contained significant similarities with the lupus LA antigen and with the helix-loop-helix region of the Myc family of proteins.