Derepression of INO1 Transcription Requires Cooperation between the Ino2p-Ino4p Heterodimer and Cbf1p and Recruitment of the

The Saccharomyces cerevisiae INO1 gene encodes the structural enzyme inositol-3-phosphate synthase for the synthesis de novo of inositol and inositol-containing phospholipids. The transcription of INO1 is completely derepressed in the absence of inositol and choline (I C). Derepression requires the binding of the Ino2p-Ino4p basic helix-loop-helix (bHLH) heterodimer to the UASINO promoter element. We report here the requirement of a third bHLH protein, centromere-binding factor 1 (Cbf1p), for the complete derepression of INO1 transcription. We found that Cbf1p regulates INO1 transcription by binding to sites distal to the INO1 promoter and encompassing the upstream SNA3 open reading frame (ORF) and promoter. The binding of Cbf1p requires Ino2p-Ino4p binding to the UASINO sites in the INO1 promoter and vice versa, suggesting a cooperative mechanism. Furthermore, Cbf1p binding to the upstream sites was required for the binding of the ISW2 chromatin-remodeling complex to the Ino2p-Ino4p-binding sites on the INO1 promoter. Consistent with this, ISW2 was also required for the complete derepression of INO1 transcription. The Saccharomyces cerevisiae INO1 (inositol-3-phosphate synthase) gene is required for the de novo synthesis of phos-phatidylinositol (PI) from glucose-6-phosphate (19, 54). Th