Identification of a mammalian RNA polymerase I holoenzyme containing components of the DNA repair/replication system

Traditional models for transcription initiation by RNA polymerase 1 include a stepwise assembly of basic transcription factors/regulatory proteins on the core promoter to form a preinitiation complex. In contrast, we have ide ntified a preassembled RNA polymerase I (RPI) complex that contains all the factors necessary and sufficient to initiate transcription from the rDNA p romoter in vitro. The purified RPI holoenzyme contains the RPI homolog of T FIID, SL-1 and the rDNA transcription terminator factor (TTF-1), but lacks UBF, an activator of rDNA transcription. Certain components of the DNA repa ir/replication system, including Ku70/80, DNA topoisomerase I and PCNA, are also associated with the RPI complex. We have found that the holoenzyme su pported specific transcription and that specific transcription was stimulat ed by the RPI transcription activator UBF. These results support the hypoth esis that a fraction of the RPI exists as a preassembled, transcriptionally competent complex that is readily recruited to the rDNA promoter, i.e. as a holoenzyme, and provide important new insights into the mechanisms govern ing initiation by RPI.