UPSTREAM BINDING-FACTOR STABILIZES RIB-1, THE TATA-BINDING-PROTEIN-CONTAINING XENOPUS-LAEVIS RNA-POLYMERASE-I TRANSCRIPTION FACTOR, BY MULTIPLE PROTEIN INTERACTIONS IN A DNA-INDEPENDENT MANNER

Initiation of RNA polymerase I transcription in Xenopus laevis require s Rib 1 and upstream binding factor (UEF). UBF and Rib 1 combine to fo rm a stable transcription complex on the Xenopus ribosomal gene promot er. Here we show that Rib 1 comprises TATA-binding protein (TBP) and T BP-associated factor components. Thus, Rib 1 is the Xenopus equivalent of mammalian SL 1. In contrast to SL I, Rib 1 is an unstable complex that readily dissociates into TBP and associated components. We identi fy a novel function for UBF in stabilizing Rib 1 by multiple protein i nteractions. This stabilization occurs in solution in a DNA-independen t manner. These results may partially explain the difference in UEF re quirement between Xenopus and mammalian systems.