Cell Cycle Regulation of E2F Site Occupation in Vivo

DNA-binding E2F complexes have been identified throughout the mammalian cell cycle, including the transcriptionally inactive complexes with pocket proteins, which occur early in the prereplicative G 1 phase of the cycle, and the transactivating free E2F, which increases in late G 1 . Here, a regulatory B- myb promoter site was shown to bind with high affinity to free E2F and to E2F-pocket protein complexes in an indistinguishable way in vitro. In contrast, in vivo footprinting with NIH 3T3 cells demonstrated E2F site occupation specifically in early G 1 , when the B- myb promoter is inactive. These observations indicate that a novel mechanism governs E2F-DNA interactions during the cell cycle and emphasize the relevance of E2F site-directed transcriptional repression.