DIMERIZATION AND HMG BOX DOMAINS 1-3 PRESENT IN XENOPUS UBF ARE SUFFICIENT FOR ITS ROLE IN TRANSCRIPTIONAL ENHANCEMENT

Transcription of Xenopus ribosomal genes by RNA polymerase I is direct ed by a stable transcription complex that forms on the gene promoter. This complex is comprised of the HMG box factor UBF and the TB P-conta ining complex Rib1. Repeated sequence elements found upstream of the r ibosomal gene promoter act as RNA polymerase I-specific transcriptiona l enhancers. These enhancers function by increasing the probability of a stable transcription complex forming on the adjacent promoter. UBF is required for enhancer function. This role in enhancement is distinc t from that at the promoter and does not involve translocation of UBF from enhancer repeats to the promoter. Here we utilize an in vitro sys tem to demonstrate that a combination of the dimerization domain of UB F and HMG boxes 1-3 are sufficient to specify its role in enhancement. We also demonstrate that the acidic C-terminus of UBF is primarily re sponsible for its observed interaction with Rib1. Thus, we have uncoup led the Rib1 interaction and enhancer functions of UBF and can conclud e that direct interaction with Rib1 is not a prerequisite for the enha ncer function of UBF.