Assembly of alternative multiprotein complexes directs rRNA promoter selectivity.
How can trans-activators with the same DNA binding specificity direct different transcriptional programs? The rRNA transcriptional apparatus offers a useful model system to address this question and to dissect the mechanisms that generate alternative transcription complexes. Here, we compare the mouse and human transcription factors that govern species-specific RNA polymerase I promoter recognition. We find that both human and mouse rRNA transcription is mediated by a specific multiprotein complex. One component of this complex is the DNA-binding transcription factor, UBF. Paradoxically, human and mouse UBF display identical DNA binding specificities even though transcription of rRNA is species specific. Promoter selectivity is conferred by a second essential factor, SL1, which, for humans, does not bind DNA independently but, instead, cooperates with UBF in the formation of high-affinity DNA-binding complexes. In contrast, mouse SL1 can selectively interact with DNA in the absence of UBF. Reconstituted transcription experiments establish that UBF and RNA polymerase I from the two species are functionally interchangeable, whereas mouse and human SL1 exhibit distinct DNA binding and transcription activities. Together, these results suggest a critical role for a specific multiprotein assembly in RNA polymerase I promoter recognition and reveal distinct mechanisms through which such complexes can generate functional diversity.
Bell
S P
SP
Howard Hughes Medical Institute, Department of Molecular and Cellular Biology, University of California, Berkeley 94720.
Jantzen
H M
HM
Tjian
R
R
D000818:Animals; D004247:DNA / Q000378:metabolism; D006801:Humans; D051379:Mice; D009097:Multienzyme Complexes / Q000378:metabolism; D011401:Promoter Regions, Genetic; D012318:RNA Polymerase I / Q000235:genetics / Q000378:metabolism; D012335:RNA, Ribosomal / Q000235:genetics*; D013045:Species Specificity; D014157:Transcription Factors / Q000378:metabolism*; D014158:Transcription, Genetic
D009097:Multienzyme Complexes; D012335:RNA, Ribosomal; D014157:Transcription Factors; D004247:DNA; D012318:RNA Polymerase I