Dimerization of yeast transcription factors Ino2 and Ino4 is regulated by precursors of phospholipid biosynthesis mediated by Opi1 repressor.

Structural genes of phospholipid biosynthesis in the yeast S. cerevisiae are activated by the heterodimeric transcription factor Ino2 + Ino4, binding to ICRE (inositol/choline-responsive element) promoter motifs. In the presence of phospholipid precursors inositol and choline, Ino2-dependent activation is inhibited by the Opi1 repressor which interacts with Ino2. In this work, we systematically investigated the importance of regulatory mechanisms possibly affecting ICRE-dependent gene expression. Autoregulatory expression of INO2, INO4 and OPI1 was abolished by promoter exchange experiments, showing that autoregulation of regulators contributes to the degree of differential gene expression but is not responsible for it. Using GFP fusion proteins, Ino2 and Ino4 were found to localize to the nucleus under conditions of repression and derepression. Interestingly, nuclear localization of Ino2 required a functional INO4 gene. Targeting of a lexA-Ino2 fusion to a heterologous promoter containing lexA operator motifs revealed a constitutive gene activation which was not influenced by phospholipid precursors. We could show that Ino2-dependent activation of a lexA-Ino4 fusion is affected by inositol and choline. Since gene activation required interaction of Ino2 and Ino4 mediated by their helix-loop-helix domains, formation/dissociation of the heterodimer must be considered as an important step of target gene regulation.