The transcription factor Opi1p regulates phospholipid synthesis in the yeast Saccharomyces cerevisiae by repressing the expression of several UAS(INO)-containing genes (e.g. INO1). Opi1p repressor activity is most active in inositol-supplemented cells. Regulation of Opi1p repressor activity is mediated by multiple phosphorylations catalyzed by protein kinases A and C. In this work, we showed that Opi1p was also phosphorylated by casein kinase II. Using purified maltose-binding protein-Opi1p as a substrate, casein kinase II activity was dose-and time-dependent and dependent on the concentrations of maltose-binding protein-Opi1p (Km = 25 microg/ml) and ATP (Km = 7 microM). Of three mutations (S10A, S38A, and S239A) in putative phosphorylation sites, 10 only the S10A mutation affected Opi1p phosphorylation. That Ser10 was a specific target of casein kinase II was confirmed by the loss of a phosphopeptide in the S10A mutant protein. The S10A mutation did not affect phosphorylation of Opi1p by either protein kinase A or protein kinase C. Likewise, phosphorylation of Opi1p by casein kinase II was not affected by mutations in protein kinase A (S31A and S251A) and protein (S10A) kinase C (S26A) phosphorylation sites. Expression of the OPI1 allele in an opi1Delta mutant attenuated (2-fold) the repressive effect of Opi1p on INO1 expression, and this effect was only observed when cells were grown in the absence of inositol. These data supported the conclusion that casein kinase II phosphorylation at Ser10 played a role in stimulating the repression of INO1 when Opi1p was not in its most active state (i.e. in inositol-deprived cells).