The negative regulator Opi1 of phospholipid biosynthesis in yeast contactsthe pleiotropic repressor Sin3 and the transcriptional activator Ino2

Structural genes of phospholipid biosynthesis in the yeast Saccharomyces ce revisiae are transcriptionally co-regulated by ICRE (inositol/choline-respo nsive element) promoter motifs. Gene activation by an ICRE is mediated by b inding of the Ino2/Ino4 transcription factor, whereas repression in the pre sence of high concentrations of inositol and choline (IC) requires an intac t Opi1 repressor. However, the mechanism of specific repression and the fun ctional interplay among these regulators remained unclear from previous wor k. Using in vivo as well as in vitro interaction assays, we show binding of the pleiotropic repressor Sin3 to the pathway-specific regulator Opi1. The paired amphipathic helix 1 (PAH1) within Sin3 and OSID (Opi1 -Sin3 interac tion domain) in the N-terminus of Opi1 were mapped as contact sites. The re gulatory significance of the Opi1-Sin3 interaction was shown by the obvious deregulation of an ICRE-dependent reporter gene in a sin3 mutant. Opi1 als o interacts with a newly identified functional domain of the transcriptiona l activator Ino2 (RID, repressor interaction domain). These results define the molecular composition of the transcription complex mediating control of ICRE-dependent genes and allow a hypothesis on the flow of regulatory info rmation in response to phospholipid precursors.