Abstract
E2F transcription factors play a critical role in the control of cell cycle progression, regulating the expression of genes involved in DNA replication, DNA repair, mitosis, and cell fate. This involves both positive-acting and negative-acting E2F proteins, the latter group including the E2F6 protein, which has been shown to function as an Rb-independent repressor of E2F-target gene transcription. In an effort to better delineate the context of E2F6 function, including the mechanisms of E2F6 functional specificity, we used chromatin immunoprecipitation assays to assess when and with what genes E2F6 associates during a cell cycle. We find that E2F6 associates specifically with the E2F target genes that are activated at G1/S; this interaction occurs during S phase of the cell cycle. In sharp contrast, E2F6 does not bind to E2F-regulated genes activated at G2/M. In the absence of E2F6, E2F4 can bind to the G1/S-regulated promoters and compensate for loss of E2F6 function. Indeed, inhibition of both E2F4 and E2F6 activity results in specific derepression of these genes during S phase. We conclude that E2F6 functions as a repressor of E2F-dependent transcription during S phase and given the specificity for the G1/S-regulated genes, we propose that E2F6 functions to distinguish G1/S and G2/M transcription during the cell cycle.