INFLUENCE OF GENE DOSAGE AND AUTOREGULATION OF THE REGULATORY GENES INO2 AND INO4 ON INOSITOL CHOLINE-REPRESSIBLE GENE-TRANSCRIPTION IN THEYEAST SACCHAROMYCES-CEREVISIAE/

Expression of structural genes of phospholipid biosynthesis in yeast i s mediated by the inositol/choline-responsive element (ICRE). ICRE-dep endent gene activation, requiring the regulatory genes INO2 and INO4, is repressed in the presence of the phospholipid precursors inositol a nd choline. INO2 and, to a less extent, INO4 are positively autoregula ted by functional ICRE sequences in the respective upstream regions. H owever, an INO2 allele devoid of its ICRE functionally complemented an INO2 mutation and completely restored inositol/choline regulation of Ino2p-dependent reporter genes. Low-level expression of INO2 and INO4 genes, each under control of the heterologous MET25 promoter, did not alter the regulatory pattern of target genes. Thus, upstream regions o f INO2 and INO4 are not crucial for transcriptional control of ICRE-de pendent genes by inositol and choline. Interestingly, over-expression of INO2, but not of INO4, counteracted repression by phospholipid prec ursors. Possibly, a functional antagonism between INO2 and a negative regulator is the key event responsible for repression or de-repression .