A novel TBP‐associated factor of SL1 functions in RNA polymerase I transcription

In mammalian RNA polymerase I transcription, SL1, an assembly of TBP and associated factors (TAFs), is essential for preinitiation complex formation at ribosomal RNA gene promoters in vitro . We provide evidence for a novel component of SL1, TAF I 41 ( MGC5306 ), which functions in Pol I transcription. TAF I 41 resides at the rDNA promoter in the nucleolus and co‐purifies and co‐immunoprecipitates with SL1. TAF I 41 immunodepletion from nuclear extracts dramatically reduces Pol I transcription; addition of SL1 restores the ability of these extracts to support Pol I transcription. In cells, siRNA‐mediated decreased expression of TAF I 41 leads to loss of SL1 from the rDNA promoter in vivo , with concomitant loss of Pol I from the rDNA and reduced synthesis of the pre‐rRNA. Extracts from these cells support reduced levels of Pol I transcription; addition of SL1 to the extracts raises the level of Pol I transcription. These data suggest that TAF I 41 is integral to transcriptionally active SL1 and imply a role for SL1, including the TAF I 41 subunit, in Pol I recruitment and, therefore, preinitiation complex formation in vivo .