THE XENOPUS RNA-POLYMERASE-I TRANSCRIPTION FACTOR, UBF, HAS A ROLE INTRANSCRIPTIONAL ENHANCEMENT DISTINCT FROM THAT AT THE PROMOTER

Repeated sequence elements found upstream of the ribosomal gene promot er in Xenopus function as RNA polymerase I-specific transcriptional en hancers. Here we describe an in vitro system in which these enhancers function in many respects as in vivo, The principal requirement for en hancer function in vitro is the presence of a high concentration of up stream binding factor (UBF), This system is utilized to demonstrate th at enhancers function by increasing the probability of a stable transc ription complex forming on the adjacent promoter, Species differences in UBF are utilized to demonstrate that enhancers do not act by recrui ting UBF to the promoter, rather UBF performs its own distinct role at the enhancers, UBF function in enhancement differs from that at the p romoter, as it is flexible with respect to both the species of UBF and the enhancer element employed, Additionally we identify a potential r ole for the mammalian UBF splice variant, UBF2, in enhancer function, We demonstrate that the TATA box binding protein (TBP)-containing comp onent of Xenopus RNA polymerase I transcription, Rib1, can interact wi th an enhancer-UBF complex, This suggests a model in which enhancers a ct by recruiting Rib1 to the promoter.