INDUCTION OF TRANSCRIPTION BY A VIRAL REGULATORY PROTEIN DEPENDS ON THE RELATIVE STRENGTHS OF FUNCTIONAL TATA BOXES

The mechanisms by which viral regulatory proteins activate the cellula r transcription apparatus without binding to specific DNA elements are not fully understood, Several lines of evidence suggest that activati on by one such regulatory protein, herpes simplex virus ICP4, could be mediated, at least in part, by TFIID. To test this model, we replaced the TATA box of the ICP4-responsive viral thymidine kinase gene with functional TATA boxes that displayed different apparent affinities for TATA-box-binding protein as measured by DNase I footprinting. We meas ured the effects of these TATA boxes on ICP4 induction by constructing ICP4 deficient recombinant viruses containing the different TATA alle les and comparing their expression in cells lacking or expressing ICP4 . Overall, ICP4 induced weak TATA boxes (those that displayed low appa rent affinity for TATA-box-binding protein and low basal expression) t he most (18- to 41-fold) and strong TATA boxes the least (7- to 10-fol d). Therefore, ICP4 induction correlated inversely with TATA box stren gth. Using a reconstituted in vitro transcription assay, we determined that the relative levels of induction by ICP4 of the different TATA a lleles were similar to those measured in vivo, suggesting that ICP4 wa s the only viral protein required for induction. These results fit a m odel in which ICP4 acts in part to enhance binding of TFIID to the TAT A box. We compare and contrast these results with those observed with the viral regulatory proteins adenovirus Ela and simian virus 40 large T antigen and the cellular coactivator PC4.