Negative regulation of phospholipid biosynthesis in Saccharomyces cerevisiae by a Candida albicans orthologue of OPI1.

Structural genes of phospholipid biosynthesis in the yeast Saccharomyces cerevisiae are coordinately regulated by a UAS element, designated ICRE (inositol/choline-responsive element). Opi1 is a negative regulator responsible for repression of ICRE-dependent genes in the presence of an excess of inositol and choline. Gene regulation by phospholipid precursors has been also reported for the pathogenic yeast Candida albicans. Screening of a data base containing raw sequences of the C. albicans genome project allowed us to identify an open reading frame exhibiting weak similarity to Opi1. Expression of the putative CaOPI1 in an opi1 mutant of S. cerevisiae could restore repression of an ICRE-dependent reporter gene. Similar to OPI1, overexpression of CaOPI1 strongly inhibited derepression of ICRE-driven genes leading to inositol-requiring transformants. Previous work has shown that Opi1 mediates gene repression by interaction with the pleiotropic repressor Sin3. The genome of C. albicans also encodes a protein similar to Sin3 (CaSin3). By two-hybrid analyses and in vitro studies for protein-protein interaction we were able to show that CaOpi1 binds to ScSin3. ScOpi1 could also interact with CaSin3, while CaOpi1 failed to bind to CaSin3. Despite of some conservation of regulatory mechanisms between both yeasts, these results suggest that repression of phospholipid biosynthetic genes in C. albicans is mediated by a mechanism which does not involve recruitment of CaSin3 by CaOpi1.