Posttranslational modification of general transcription factors may be an important mechanism for global gene regulation. The general transcription factor IIA (TFIIA) binds to the TATA binding protein (TBP) and is essential for high-level transcription mediated by various activators. Modulation of the TFIIA-TBP interaction is a likely target of transcriptional regulation. We report here that Toa1, the large subunit of yeast TFIIA, is phosphorylated in vivo and that this phosphorylation stabilizes the TFIIA-TBP-DNA complex and is required for high-level transcription. Alanine substitution of serine residues 220, 225, and 232 completely eliminated in vivo phosphorylation of Toa1, although no single amino acid substitution of these serine residues eliminated phosphorylation in vivo. Phosphorylated TFIIA was 30-fold more efficient in forming a stable complex with TBP and TATA DNA. Dephosphorylation of yeast-derived TFIIA reduced DNA binding activity, and recombinant TFIIA could be stimulated by in vitro phosphorylation with casein kinase II. Yeast strains expressing the toa1 S220/225/232A showed reduced high-level transcriptional activity at the URA1, URA3, and HIS3 promoters but were viable. However, S220/225/232A was synthetically lethal when combined with an alanine substitution mutation at W285, which disrupts the TFIIA-TBP interface. Phosphorylation of TFIIA could therefore be an important mechanism of transcription modulation, since it stimulates TFIIA-TBP association, enhances high-level transcription, and contributes to yeast viability.