Regulation of the yeast INO1 gene: The products of the IN02, IN04 and OPI1regulatory genes are not required for repression in response to inositol

The ino2 Delta, ino4 Delta, opi1 Delta, and sin3 Delta mutations all affect expression of INO1, a structural gene for inositol-1-phosphate synthase. T hese same mutations affect other genes of phospholipid biosynthesis that, l ike INO1, contain the repeated element UAS(INO) (consensus 5' GATGTGAAAT 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 str ains carrying an ino2 Delta or ino4 Delta mutation, or both, failed to grow in medium lacking inositol. However, when grown in liquid culture in mediu m containing limiting amounts of inositol, the opi1 Delta ino4 Delta strain exhibited a level of INO1 expression comparable to, or higher than, the wi ld-type strain growing under the same conditions. Furthermore, INO1 express ion in the opi1 Delta ino4 Delta strain was repressed in cells grown in med ium fully supplemented with both inositol and choline. Similar results were obtained using the opi1 Delta ino2 Delta ino4 Delta 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 leve l of INO1 expression in an antagonistic manner, they do not appear to be re sponsible for transmitting the signal that leads to repression of INO1 in r esponse to inositol. Various models for Opi1p function were tested and no e vidence for binding of Opi1p to UAS(INO), or to Ino2p or Ino4p, was obtaine d.