The retinoblastoma protein selectively represses E2F1 targets via a TAAC DNA element during cellular senescence

The retinoblastoma (Rb) protein mediates heterochromatin formation at the promoters of E2 transcription factor 1 (E2F1) target genes, such as proliferating cell nuclear antigen and cyclin A2 (CCNA2), and represses these genes during cellular senescence. However, the selectivity of Rb recruitment is still not well understood. Here, we demonstrate that a senescence-associated gene is a direct target of E2F1 and is also repressed by heterochromatin in senescent cells. In contrast, ARF and p27(KIP1), which are also E2F1 targets, are not repressed by Rb and heterochromatin formation. By comparing the promoter sequences of these genes, we found a novel TAAC element that is present in the cellular senescence-inhibited gene, proliferating cell nuclear antigen, and CCNA2 promoters but absent from the ARF and p27(KIP1) promoters. This TAAC element associates with Rb and is required for Rb recruitment. We further determined that TAAC element-mediated Rb association requires the E2F1 binding site, but not E2F1 protein. These results provide a novel molecular mechanism for the different expression patterns of E2F1 targets and afford new mechanistic insight regarding the selectivity of Rb-mediated heterochromatin formation and gene repression during cellular senescence.