HEAT-SHOCK FACTOR INCREASES THE REINITIATION RATE FROM POTENTIATED CHROMATIN TEMPLATES

Transcription by RNA polymerase II is highly regulated at the level of initiation and elongation. Well-documented transcription activation m echanisms, such as the recruitment of TFIID and TFIIB, control the ear ly phases of preinitiation complex formation, The heat shock genes pro vide an example for transcriptional regulation at a later step: in nuc lei TFIID can be detected at the TATA box prior to heat induction, Usi ng cell-free systems for chromatin reconstitution and transcription, w e have analyzed the mechanisms by which heat shock factor (HSF) increa ses transcription of heat shock genes in chromatin, HSF affected trans cription of naked DNA templates in multiple ways: (i) by speeding up t he rate of preinitiation complex formation, (ii) by increasing the num ber of productive templates, and (iii) by increasing the reinitiation rate, Under the more physiological conditions of potentiated chromatin templates, HSF affected only the reinitiation rate, Activator-depende nt reinitiation of transcription, obviating the slow assembly of the T FIID-TFIIA complex on a promoter, may be especially crucial for genes requiring a fast response to inducers.