Question

Draw a genetic switch containing two genes : R and L, where each gene represses the other gene.

Answer 1

the mechanism of repression differs from that of cell cycle-regulated genes. We find that two different activities are involved in their regulation and that in proliferating cells, both are required to maintain repression. First, dE2F2/RBF and dREAM/MMB employ histone deacetylase (HDAC) activities at promoter regions.

* an unconventional mechanism of repression by the Polycomb group (PcG) protein Enhancer of zeste [E(Z)], which is involved in silencing of these genes through the dimethylation of histone H3 Lys27 at nucleosomes located downstream of the transcription start sites (TSS).

The retinoblastoma protein (pRB) is a critical regulator of cell division, cell death, and differentiation in metazoans, and its activity is altered in most human tumors

The best understood property of pRB is its ability to modulate the action of the E2F family of transcription factors and to regulate cell cycle progression. pRB and the related proteins p107 and p130, collectively referred to as “pocket proteins,” or RB family proteins (RB), bind to the heterodimeric E2F/DP factors and provide a module of transcriptional regulation that couples the expression of many genes with cell cycle progression. In quiescent cells, E2F and pocket proteins form repressive complexes that prevent the transcription of genes required for S-phase entry. This repression is then relieved at the G1-to-S transition by the activity of cyclin-dependent kinases (Cdk). At the promoters of cell cycle-regulated genes, repressive E2F/RB complexes are replaced by activating E2Fs, and this allows for the coordinated expression of many genes required for cell division.

Mechanism 2.

The mechanism(s) of action of dREAM/MMB remains uncertain. the role of the dREAM/MMB complex in RB-mediated repression at developmentally regulated genes by examining the chromatin modifications at these genes and their dependence on E2F/RB/dREAM/MMB. The one of which involves HDAC activity and histone deacetylation of nucleosomes at promoter regions.

and the other the activity of a Polycomb group protein, Enhancer of zeste [E(Z)], and the dimethylation of histone H3 lysine 27 (H3K27me2) at nucleosomes located downstream from the transcription start site.

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