E2F-MEDIATED GROWTH-REGULATION REQUIRES TRANSCRIPTION FACTOR COOPERATION

Previous studies have indicated that the presence of an E2F site is no t sufficient for G(1)/S phase transcriptional regulation, For example, the E2F sites in the E2F1 promoter are necessary, but not sufficient, to mediate differential promoter activity in G(0) and S phase, We hav e now utilized the E2F1 minimal promoter to test several hypotheses th at could account for these observations, To test the hypothesis that G (1)/S phase regulation is achieved via E2F-mediated repression of a st rong promoter, a variety of transactivation domains were brought to th e E2F1 minimal promoter, Although many of these factors caused increas ed promoter activity, growth regulation was not observed, suggesting t hat a general repression model is incorrect. However, constructs havin g CCAAT or YY1 sites or certain GC boxes cloned upstream of the E2F1 m inimal promoter displayed E2F site-dependent regulation, Further analy sis of the promoter activity suggested that E2F requires cooperation w ith another factor to activate transcription in S phase, However, we f ound that the requirement for E2F to cooperate with additional factors to achieve growth regulation could be relieved by bringing the E2F1 a ctivation domain to the promoter via a Gal4 DNA binding domain, Our re sults suggest a model that explains why some, but not all, promoters t hat contain E2F sites display growth regulation.