| RB and E2F proteins play important roles in the regulation of cell division, cell death and development, by controlling the expression of genes involved in these processes. The mechanisms of repression by pRB have been extensively studied at cell cycle regulated promoters. However, little is known about developmentally regulated E2F/RB genes. Here I have taken advantage of the simplicity of the E2F/RB pathway in flies, and inspected the regulation of differentiation-specific target genes.;These genes are repressed by dE2F2/RBF and a recently identified RB-containing complex, dREAM, in a cell type- and cell cycle-independent manner. Two different types of activities are involved in their regulation. First, I find that dREAM employs histone deacetylase (HDAC) activities at promoter regions and that HDACs are required to maintain repression. Second, I find that the Polycomb Group (PcG) protein, Enhancer of zeste - E(Z), is involved in silencing of these genes through the di-methylation of histone H3 Lys27 at nucleosomes located downstream of the transcription start sites (TSS). While HDAC activity is also involved in the regulation of cell cycle dependent E2F transcription, E(Z) functions at differentiation-specific target genes only, indicating that the two groups of genes are regulated in a distinct manner.;The differentiation-specific genes are also regulated differently from cell cycle-related E2F targets, in a way that they do not depend on E2F-activation. E2F/RB repression is maintained throughout the cell cycle. I demonstrated that the dREAM complex is required for dE2F2 binding at differentiation-specific, but not cell cycle-regulated E2F/RB target gene promoters. Especially, dREAM complex is necessary for dE2F2/RBF binding in S-phase.;Taking together, my results demonstrate that dREAM plays a dual role in the regulation of differentiation-specific genes. dREAM complex is required for the stability of dE2F2/RBF complexes at these promoters during S-phase, and also for the repression mechanisms employed at these genes. |
