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 res pect 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 p olypeptide of 434 amino acids with a sequence similarity to yeast Pol I 49 kDa subunit. Anti-PAF53 antibody does not block the random transc ription activity of Pol I, but blocks specific transcription from mous e ribosomal RNA promoter, demonstrating the requirement of PAF53 in th e accurate initiation of Pol I transcription, Moreover, PAF53 interact ed with mouse UBF in vitro, as revealed by Far-Western blotting and GS T pull down assays, These results, together with the accumulation of P AF53 in the nucleolus of growing cells, suggest that PAF53 is involved in the formation of the initiation complex at the promoter by mediati ng the interaction between Pol I and UBF for the active rRNA synthesis .