A ROLE FOR ACTIVATOR-MEDIATED TFIIB RECRUITMENT IN DIVERSE ASPECTS OFTRANSCRIPTIONAL REGULATION

Background: Transcription by RNA polymerase II in eukaryotic cells req uires the ordered assembly of general transcription factors on the pro moter to form a preinitiation complex. Transcriptional activator prote ins (activators) stimulate transcription by increasing the rate and/or extent of preinitiation complex assembly. We have shown previously th at acidic activators increase the stable association of TFIIB on the p romoter, a process we refer to as 'recruitment'. In this study, we pro vide evidence that diverse activators facilitate TFIIB assembly by a r elated mechanism. We then investigate the activator-mediated assembly of TFIIB with regard to two aspects of transcription: the distance-dep endence of activator function, and reinitiation. Results: We have prev iously described amino-acid-substitution mutants of TFIIB that are abl e to support an activator-independent basal level of transcription but do not respond to acidic activators. We now show that these mutants a lso do not respond to other classes of activators. We demonstrate that this defect is due to a failure of the activators to recruit the muta nt TFIIB to the promoter. Activators often lose activity as their dist ance from the initiation site is increased. We show that this impaired transcriptional activity correlates with a decrease in TFIIB recruitm ent. Finally, we find that following the initiation of transcription, TFIIB dissociates from the promoter, requiring the activator-mediated reassembly of TFIIB in the preinitiation complex for each new round of transcription. Conclusion: We have provided evidence that diverse act ivators recruit TFIIB to the promoter by a related mechanism. This cen tral step in transcriptional activation is sensitive to promoter archi tecture, and is required for each new round of transcription.