Acetylation of TAF(I)68, a subunit of TIF-IB/SL1, activates RNA polymeraseI transcription

Mammalian rRNA genes are preceded by a terminator element that is recognize d by the transcription termination factor TTF-I. In exploring the functiona l significance of the promoter-proximal terminator, we found that TTF-I ass ociates with the p300/CBP-associated factor PCAF, suggesting that TTF-I may target histone acetyltransferase to the rDNA promoter, We demonstrate that PCAF acetylates TAF(I)68, the second largest subunit of the TATA box-bindi ng protein (TBP)-containing factor TIF-IB/SL1, and acetylation enhances bin ding of TAF(I)68 to the rDNA promoter. Moreover, PCAF stimulates RNA polyme rase I(Pol I) transcription in a reconstituted in vitro system. Consistent with acetylation of TIF-IB/ SL1 being required for rDNA transcription, the NAD(+)-dependent histone deacetylase mSir2a deacetylates TAF(I)68 and repre sses Pol I transcription, The results demonstrate that acetylation of the b asal pol I transcription machinery has functional consequences and suggest that reversible acetylation of TIF-IB/SL1 may be an effective means to regu late rDNA transcription in response to external signals.