[Interaction of E2F/Rb family members with factor binding to co-repressor element on B-myb and E2F1 promoters]

The E2F transcription factor plays an important role in controlling the expression of genes required for cell cycle progression. The transcription of a number of these genes, including E2F1 and B-myb, is repressed in G0/early G1 at E2F DNA binding sites mediated by interaction of E2F with the Rb family member proteins. It was shown that a corepressor element CHR, which was originally identified in the B-myb promoter, is also responsible for the repression of the E2F1 promoter. The mutation of the CHR element adjacent to E2F sites leads to a derepression of the E2F1 promoter in quiescent cells. The CHR-mutated promoter is activated by the E2F family of proteins (E2F1, E2F2, E2F3, and E2F4) but unable to be repressed by any of the Rb family members (Rb, p107, and p130) to the level of the wild-type promoter activity in G0, indicating that the repression by the Rb family members is required for the corepressor element. Moreover, it was shown that a factor specifically bound to the CHR element is co-purified with E2F by DNA affinity purification and co-immunoprecipitated with E2F4 and the Rb family members. These results seggested that E2F and the Rb family member proteins regulate the transcription of the E2F1 and B-myb genes by associating with an additional corepressor protein.