RNA contacts the two large polymerase subunits and a 52-kDa polypeptide in nucleolar RNA polymerase I transcribing complexes.

The interactions between nascent RNA and RNA polymerase I-transcribing complexes were studied by photoaffinity labeling in HeLa nucleoli. Specific labeling of enzyme subunits Ia, Ib, and of a 52-kDa polypeptide was observed. The 52-kDa polypeptide was shown to be associated with the transcribing complex by immunoprecipitation with RNA polymerase I antibodies and by glycerol gradient centrifugation. Transcripts that were kept short by inclusion of a chain-terminating nucleotide or with short incubation times contacted mainly subunits Ia and Ib. Decreasing chain terminator concentration or increasing incubation time resulted in additional labeling of the 52-kDa polypeptide. These data indicate that the 52-kDa polypeptide contacts nascent RNA after it has left the Ia-Ib domain.