Chromatin is permissive to TATA-binding protein (TBP)-mediated transcription initiation

Preinitiation complex assembly is nucleated by the binding of TFIID to the promoters of protein coding genes transcribed by RNA polymerase II, TFIID i s comprised of the TATA-binding protein (TBP) and TBP-associated factors (T AF(II)s), We investigated the transcription properties of TBP and TFIID on chromatin templates. On naked templates both TBP and purified TFIID are abl e to initiate basal transcription. However, on chromatin templates only TBP mediates transcription initiation in a heat-treated extract, whereas TFIID does not. Moreover, TBP-mediated chromatin transcription is blocked in a n ontreated extract. These observations suggest that a chromatin-targeted rep ressor is present in crude extracts and that chromatin per se is not refrac tory to transcription mediated by TBP, As TSP can function through TAF(II)- independent and TAF(II)-dependent pathways, the repression of TBP-mediated basal transcription may be an additional level to the control of Pol II tra nscription initiation on chromatin.