Figure 9.
A model of regulation of the POLE2 promoter. The region containing Sp1, E2F and NF1 elements is occupied by multiprotein complexes in both quiescent and cycling cells. A dramatic increase in promoter activity takes place during G0→G1. We propose that two E2F–pocket protein complexes repress transcription in quiescent cells, one containing p107 and the other pRb. After a mitogenic stimulus, cyclinD-Cdk4/6 phosphorylates the pocket proteins, disrupting these complexes. The level of promoter activity in cycling cells may result from modulation of species of bound E2Fs and pocket proteins. Sp1 is essential for promoter activity in cycling cells and may interact with E2F1 to form a repressive complex in quiescent cells.