FUNCTIONAL DIFFERENCES BETWEEN THE 2 SPLICE VARIANTS OF THE NUCLEOLARTRANSCRIPTION FACTOR UBF - THE 2ND HMG BOX DETERMINES SPECIFICITY OF DNA-BINDING AND TRANSCRIPTIONAL ACTIVITY

The nucleolar transcription factor UBF consists of two proteins, UBF1 and UBF2, which originate by alternative splicing. Here we show that d eletion of 37 amino acids within the second of five HMG box moths in U BF2 is important for the dual role of UBF as transcriptional activator and antirepressor. UBF1 is a potent antirepressor and transcriptional activator, whereas the ability of UBF2 to counteract histone H1-media ted repression and to stimulate ribosomal gene transcription both in v ivo and in vitro is at least one order of magnitude lower. The differe nce in transcriptional activity between UBF1 and UBF2 is due to their different binding to the ribosomal gene promoter and enhancer. Apparen tly, the presence of an intact HMG box2 modulates the sequence-specifi c binding of UBF to rDNA control elements. However, the interaction of UBF with rDNA does not entirely depend on sequence recognition. Both UBF isoforms bind efficiently to four-way junction DNA, indicating tha t they recognize defined DNA structures rather than specific sequences . The results demonstrate that the HMG boxes are functionally diverse and that HMG box2 plays an important role in specific binding of UBF t o rDNA.