The coactivator p15 (PC4) initiates transcriptional activation during TFIIA-TFIID-promoter complex formation.

We have analyzed the mechanisms underlying stimulation of transcription by the activator GAL4-AH and the recombinant coactivator p15 (PC4). We show that p15 binds to both double-stranded and single-stranded DNA. Analyses of deletion mutants correlates binding to double-stranded DNA with the ability to mediate activator-dependent transcription. Consistent with this finding, phosphorylation of p15 by casein kinase II inhibits binding to double-stranded DNA and the activity of p15. The functional characterization suggests interactions of p15 with both DNA and components of the TFIID complex. GAL4-AH functions in concert with p15 during formation of TFIIA-TFIID-promoter (DA) complexes, as concluded from order-of-addition experiments. At limiting TFIID concentrations, the number of DA complexes is enhanced. The activator also stimulates transcription moderately after DA complex formation, independently of the concentrations of general transcription factors.