A dominant mutation of Saccharomyces cerevisiae, CSE1, caused a decrease in the expression of the INO1 gene product, inositol-1-phosphate synthase. The residual activity was completely repressed by the addition of choline to the medium. A mutant carrying this mutation could not grow in the presence of choline unless inositol was added to the medium. Here we report a suppressor gene of the CSE1 mutation, SCS1 (suppressor of CSE1), which was cloned by complementation of CSE1 with a wild-type multicopy yeast genomic library. The cloned SCS1 gene contained an open reading frame which encoded 304 amino acid residues with a calculated molecular mass of 34,234 Da, and the sequence coincided with that of the INO2 gene. An scs1/ino2 null mutant constructed by gene replacement was viable, but auxotrophic for inositol and choline, and used for determination of the mRNA levels of various phospholipid-synthesizing enzymes. In agreement with the reported data for ino2 mutants the disruptant showed decreased expression of the INO1 and PSS genes, which are known to be regulated by inositol and choline. In addition, we newly found that the disruption of SCS1/INO2 also caused a decrease in the expression of the CKI, PEM1, and PEM2 genes, which we previously showed to belong to the inositol-choline-regulated gene family. These results confirm and strengthen the conclusion that the SCS1/INO2 gene is required for expression of inositol-choline-regulated genes in phospholipid synthesis.
