TIF-IE, a transcription factor necessary for ribosomal RNA template commitment

Abstract

In the small, free-living amoeba, Acanthamoeba castellanii, ribosomal RNA (rRNA) transcription requires in addition to RNA polymerase I (Pol I), a single DNA-binding factor, Transcription Initiation Factor-IB (TIF-IB). TIF-IB is a multimeric protein that contains TATA-binding protein (TBP) and four TBP-associated factors (TAFIS) that are specific for Pol I transcription with relative molecular weights of 91, 96, 99 and 145. TIF-IB is required for accurate and promoter-specific initiation of rRNA transcription, recruiting and positioning the polymerase on the start site by protein-protein interaction. In A. castellanii, partially purified TIF-IB can form a persistent complex with the rDNA promoter while homogeneous TIF-IB cannot. Another factor, TIF-IE, is required along with homogeneous TIF-IB for the formation of a stable complex on the rDNA core promoter. TIF-IE by itself, however, does not bind to the rDNA promoter and thus differs in mechanism from the Upstream Binding Factor (UBF) and Upstream Activating Factor (UAF) that carry out similar complex stabilizing functions in vertebrates and yeast, respectively. In addition to its presence in impure TIF-IB, TIF-IE has been found in highly purified fractions of Pol I, with which it associates, and can be partially separated from the polymerase by rate zonal sedimentation. Renaturation of polypeptides excised from SDS-polyacrylamide gels showed that a 141 kDa polypeptide possesses all the known activities of TIF-IE. In other eukaryotic systems, no obviously similar factor to TIF-IE has been reported.