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  • Title: Identification of a mammalian RNA polymerase I holoenzyme containing components of the DNA repair/replication system.
    Author: Hannan RD, Cavanaugh A, Hempel WM, Moss T, Rothblum L.
    Journal: Nucleic Acids Res; 1999 Sep 15; 27(18):3720-7. PubMed ID: 10471742.
    Abstract:
    Traditional models for transcription initiation by RNA polymerase I include a stepwise assembly of basic transcription factors/regulatory proteins on the core promoter to form a preinitiation complex. In contrast, we have identified a preassembled RNA polymerase I (RPI) complex that contains all the factors necessary and sufficient to initiate transcription from the rDNA promoter in vitro. The purified RPI holoenzyme contains the RPI homolog of TFIID, SL-1 and the rDNA transcription terminator factor (TTF-1), but lacks UBF, an activator of rDNA transcription. Certain components of the DNA repair/replication system, including Ku70/80, DNA topoisomerase I and PCNA, are also associated with the RPI complex. We have found that the holo-enzyme supported specific transcription and that specific transcription was stimulated by the RPI transcription activator UBF. These results support the hypothesis 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 governing initiation by RPI.
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