E2F-mediated growth regulation requires transcription factor cooperation

Previous studies have indicated that the presence of an E2F site is not 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 have now utilized the E2F1 minimal promoter to test several hypotheses that could account for these observations, To test the hypothesis that G(1)/S phase regulation is achieved via E2F-mediated repression of a strong promoter, a variety of transactivation domains were brought to the E2F1 minimal promoter, Although many of these factors caused increased promoter activity, growth regulation was not observed, suggesting that a general repression model is incorrect. However, constructs having CCAAT or YY1 sites or certain GC boxes cloned upstream of the E2F1 minimal promoter displayed E2F site-dependent regulation, Further analysis of the promoter activity suggested that E2F requires cooperation with another factor to activate transcription in S phase, However, we found that the requirement for E2F to cooperate with additional factors to achieve growth regulation could be relieved by bringing the E2F1 activation domain to the promoter via a Gal4 DNA binding domain, Our results suggest a model that explains why some, but not all, promoters that contain E2F sites display growth regulation.