Identification of functional targets of the Zta transcriptional activator by formation of stable preinitiation complex intermediates

Transcriptional activator proteins stimulate the formation of a preinitiation complex that may be distinct from a basal-level transcription complex in its composition and stability. Components of the general transcription factors that form activator-dependent stable intermediates were determined by the use of Sarkosyl and oligonucleotide challenge experiments. High-level transcriptional activation by the Epstein-Barr virus-encoded Zta protein required an activity in the TFIID fraction that is distinct from the TATA-binding protein (TBP) and the TBP-associated factors. This additional activity copurifies with and is likely to be identical to the previously defined coactivator, USA (M. Meisterernst, A. L. Roy, H. M. Lieu, and R. G. Roeder, Cell 66:981-994, 1991). The formation of a stable preinitiation complex intermediate resistant to Sarkosyl required the preincubation of the promoter DNA with Zta, holo-TFIID (TBP and TBP-associated factors), TFIIB, TFIIA, and the coactivator USA. The formation of a Zta response element-resistant preinitiation complex required the preincubation of promoter DNA with Zta, holo-TFIID, TFIIB, and TFIIA. Agarose gel electrophoretic mobility shift showed that a preformed Zta-holo-TFIID-TFIIA complex was resistant to Sarkosyl and to Zta response element oligonucleotide challenge. DNase I footprinting suggests that only Zta, holo-TFIID, and TFIIA make significant contacts with the promoter DNA. These results provide functional and physical evidence that the Zta transcriptional activator influences at least two distinct steps in preinitiation complex assembly, the formation of the stable holo-TFIID-TFIIA-promoter complex and the subsequent binding of TFIIB and a USA-like coactivator.