SPECIES-SPECIFICITY OF RIBOSOMAL GENE-TRANSCRIPTION - A FACTOR ASSOCIATED WITH HUMAN RNA-POLYMERASE-I PREVENTS TRANSCRIPTION OF MOUSE RDNA

An intrinsic property of class I gene transcription by RNA polymerase I (Pol I) is the species specificity of the initiation reaction. Previ ous studies have demonstrated that species-specific rDNA promoter reco gnition is brought about by a TBP-TAF complex, termed TIF-IB in mouse and SL1 in man. We have compared the ability of affinity-purified TIF- IB and SL1 to direct transcription from the homologous rDNA template b oth in a reconstituted transcription system and in nuclear extracts pr epared from mouse and human cells. We show that Pol I from both specie s and the individual transcription factors, with the exception of TIF- IB/SL1, are functionally interchangeable in the reconstituted transcri ption system containing purified proteins. In nuclear extracts, howeve r, species-specific differences are obvious. Whereas SL1 reprograms a heterologous mouse extract to recognize the human promoter. TLF-IB fai ls to reprogram a human extract unless it is complemented with mouse P ol I. Crude human, but not mouse, Pol I exhibits species-specific diff erences that disappear after purification. We propose that in extracts and less purified fractions human Pol I exists as 'holoenzyme' contai ning associated protein(s) that prevent assembly of TIF-IB-directed in itiation complexes at the murine rDNA promoter.