Two E2F sites control growth-regulated and cell cycle-regulated transcription of the Htf9-a/RanBP1 gene through functionally distinct mechanisms

The gene encoding Ran-binding protein 1 (RanBP1) is transcribed in a cell c ycle-dependent manner. The RanBP1 promoter contains two binding sites for E 2F factors, named E2F-c, located proximal to the transcription start, and E 2F-b, falling in a more distal promoter region, We have now induced site-di rected mutagenesis in both sites. We have found that the distal E2F-b site, together with a neighboring Spl element, actively controls up-regulation o f transcription in S phase. The proximal E2F-c site plays no apparent role in cycling cells yet is required for transcriptional repression upon growth arrest. Protein binding studies suggest that each E2F site mediates specif ic interactions with individual E2F family members. In addition, transient expression assays with mutagenized promoter constructs indicate that the fu nctional role of each site is also dependent on its position relative to ot her regulatory elements in the promoter context. Thus, the two E2F sites pl ay opposite genetic functions and control RanBP1 transcription through dist inct molecular mechanisms.