RNA polymerase I associated factor 53 binds to the nucleolar transcription factor UBF and functions in specific rDNA transcription.

Mouse RNA polymerase I (Pol I) has, besides its 11 bona fide subunits, three polymerase associated factors, termed PAF53, 51 and 49 with respect to the size of each molecule. In order to analyze the function of PAFs, cDNA encoding PAF53 was isolated using an oligonucleotide probe derived from an oligopeptide sequence. The cDNA of PAF53 predicts a polypeptide of 434 amino acids with a sequence similarity to yeast Pol 1 49 kDa subunit. Anti-PAF53 antibody does not block the random transcription activity of Pol I, but blocks specific transcription from mouse ribosomal RNA promoter, demonstrating the requirement of PAF53 in the accurate initiation of Pol I transcription. Moreover, PAF53 interacted with mouse UBF in vitro, as revealed by Far-Western blotting and GST pull down assays. These results, together with the accumulation of PAF53 in the nucleolus of growing cells, suggest that PAF53 is involved in the formation of the initiation complex at the promoter by mediating the interaction between Pol I and UBF for the active rRNA synthesis.