TBP-ASSOCIATED FACTORS INTERACT WITH DNA AND GOVERN SPECIES-SPECIFICITY OF RNA-POLYMERASE-I TRANSCRIPTION

Unlike genes transcribed by RNA polymerases II and III, transcription by RNA polymerase I is highly species-specific. Ribosomal promoter sel ectivity is brought about by a multisubunit transcription factor (SL1/ TIF-IB) which consists of the TATA-binding protein (TBP) and three TBP -associated factors (TAFs). To determine the basis for the inability o f SL1/TIF-IB to recognize heterologous rDNA, the transcriptional prope rties and the subunit composition of the murine and the human factor, as well as a chimeric complex containing epitope-tagged human TBP and murine TAFs, have been compared. We show that TBP can be exchanged bet ween the human and mouse factor indicating that the variable N-termina l domain of TBP does not play a significant role in rDNA promoter sele ctivity. Instead, DNA binding is brought about by the TAFs. UV crossli nking experiments demonstrate that binding to the ribosomal gene promo ter is mediated by two TAFs (TAF(I)48 and TAF(I)68) which have the sam e electrophoretic mobility in the human and mouse factor. The largest TAF is different in both species and is suggested to play a role in th e species-specific assembly of productive preinitiation complexes. Thu s, evolutionary changes of rDNA promoter sequences have been accompani ed by changes in specific TAFs.