Regulation of TFII-I activity by phosphorylation.

The transcription factor TFII-I binds to distinct promoter sequences including an initiator element in several eukaryotic genes. Here we demonstrate that TFII-I is phosphorylated in vivo at serine/threonine and tyrosine residues in the absence of any apparent extracellular signals. This "basal" phosphorylation of TFII-I is not required and does not affect its specific DNA binding, but is critical for its in vitro transcriptional properties via the Vbeta promoter. To better assess the functional role of phosphorylation in regulating TFII-I activity, we focused on tyrosine phosphorylation of TFII-I. Ectopically expressed recombinant TFII-I, like its native counterpart, exhibits tyrosine phosphorylation in the absence of distinct extracellular signals. More important, mutation of a potential consensus tyrosine phosphorylation site in TFII-I leads to severe reduction in its basal transcriptional activation of the Vbeta promoter in vivo. Taken together, these data suggest that tyrosine phosphorylation of TFII-I is important for its initiator-dependent transcriptional activity.