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 requires Rib 1 and upstream binding factor (UBF). UBF and Rib 1 combine to form a stable transcription complex on the Xenopus ribosomal gene promoter. Here we show that Rib I comprises TATA-binding protein (TBP) and TBP-associated factor components. Thus, Rib 1 is the Xenopus equivalent of mammalian SL 1. In contrast to SL 1, Rib 1 is an unstable complex that readily dissociates into TBP and associated components. We identify a novel function fur UBF in stabilizing Rib I by multiple protein interactions. This stabilization occurs in solution in a DNA-independent manner. These results may partially explain the difference in UBF requirement between Xenopus and mammalian systems.