Figure 2: Human G1/S Transition and Replication Stress Response.
A) Prior to S-phase, E2F transcription is inhibited by Rb binding. Upon entrance to S-phase, G1 cyclin/CDK complex phosphorylates Rb, releasing E2F1 to permit transcription. E2F6 then inhibits transcription of G1/S targets. B) Upon replication stress, ATR activates Chk1 to phosphorylate E2F6, promoting the transcription of E2F1 targets.