A TBP-CONTAINING MULTIPROTEIN COMPLEX (TIF-IB) MEDIATES TRANSCRIPTIONSPECIFICITY OF MURINE RNA POLYMERASE-I

TIF-IB is a transcription factor which interacts with the mouse riboso mal gene promoter and nucleates the formation of an initiation complex containing RNA polymerase I (Pol I). We have purified this factor to near homogeneity and demonstrate that TIF-IB is a large complex (<200 kDa) which contains several polypeptides. One of the subunits present in this protein complex is the TATA-binding protein (TBP) as revealed by copurification of TIF-IB activity and TBP over different chromatogr aphic steps including immunoaffinity purification. In addition to TBP, three tightly associated proteins (TAFs-I) with apparent molecular we ights of 95, 68, and 48 kDa are contained in this multimeric complex. This subunit composition is similar-but not identical-to the analogous human factor SL1. Depletion of TBP from TIF-IB-containing fractions b y immunoprecipitation eliminates TIF-IB activity. Neither TBP alone no r fractions containing other TBP complexes are capable of substituting for TIF-IB activity. Therefore, TIF-IB is a unique complex with Pol I -specific TAFs distinct from other TBP-containing complexes. The ident ification of TBP as an integral part of the murine rDNA promoter-speci fic transcription initiation factor extends the previously noted simil arity of transcriptional initiation by the three nuclear RNA polymeras es and underscores the importance of TAFs in determining promoter spec ificity.