Figure 1

Cell cycle and transcription factor complexes. The protein complexes binding to DNA change during the cell cycle. Gene expression is repressed in the early phases of the cell cycle and becomes activated during the later phases. For this change, E2F and CHR (cell cycle genes homology region) promoter elements switch from repressor to activator sites. In G₀ and early G₁ phase the DREAM complex binds E2F, CHR, CDE (cell cycle-dependent element), and CLE (CHR-like element) sites to repress transcription. In G₂ phase and mitosis transcriptional repression is released and activation occurs via CHR sites. Only promoters with CHR sites can bind the MuvB-based complexes MMB (B-MYB-MuvB), FOXM1-MMB and FOXM1-MuvB. The MuvB core complex is composed of LIN9, LIN37, LIN52, LIN54 and RBBP4 proteins. LIN54 is the component which binds to CHR elements. For the switch from repressing to activating complexes, B-MYB and FOXM1 are recruited to the MuvB core when E2F4-5/DP and p107/p130 dissociate from the complex. B-MYB-MuvB (MMB), FOXM1-MMB and FOXM1-MuvB complexes serve as activators of late cell cycle genes which carry functional CHR elements. Early cell cycle genes with maximum expression in the S phase are activated by E2F1-3/DP heterodimers through E2F sites