Regulation of the Yeast INO1 Gene: The Products of the INO2, INO4 and OPI1 Regulatory Genes Are Not Required for Repression in Response to Inositol

The ino2Δ, ino4Δ, opi1Δ, and sin3Δ mutations all affect expression of INO1, a structural gene for inositol-1-phosphate synthase. These same mutations affect other genes of phospholipid biosynthesis that, like INO1, contain the repeated element UASINO (consensus 5′ CATGTGAAAT 3′). In this study, we evaluated the effects of these four mutations, singly and in all possible combinations, on growth and expression of INO1. All strains carrying an ino2Δ or ino4Δ mutation, or both, failed to grow in medium lacking inositol. However, when grown in liquid culture in medium containing limiting amounts of inositol, the opi1Δ ino4Δ strain exhibited a level of INO1 expression comparable to, or higher than, the wild-type strain growing under the same conditions. Furthermore, INO1 expression in the opi1Δ ino4Δ strain was repressed in cells grown in medium fully supplemented with both inositol and choline. Similar results were obtained using the opi1Δ ino2Δ ino4Δ strain. Regulation of INO1 was also observed in the absence of the SIN3 gene product. Therefore, while Opi1p, Sin3p, and the Ino2p/Ino4p complex all affect the overall level of INO1 expression in an antagonistic manner, they do not appear to be responsible for transmitting the signal that leads to repression of INO1 in response to inositol. Various models for Opi1p function were tested and no evidence for binding of Opi1p to UASINO, or to Ino2p or Ino4p, was obtained.