Gene silencing model

Fig. 22.1. A model of gene silencing. Long, double-stranded RNAs (dsRNAs) are processed into 20-26 nucleotide small interfering RNAs (siRNAs) by Dicer (Step 1). The siRNAs associate with an RNA-induced silencing complex (RISC, Step 2), unwinding and activating in the process (Step 3). The antisense strand of the siRNA guides the RISC to complementary mRNA molecules (Step 4), which are cleaved and destroyed (Step 5). Sense and antisense RNA strands are indicated by thick and thin lines, respectively.

Figure 2 A model for RAR/RXR acting in concert with coactivator or corepressor complex for gene activation or silencing. In the presence of ligands (+RA), the holo-receptor pair binds to the RA response element (RARE) and recruits coactivator complex, which encodes histone acetyl transferase (HAT) activity. HAT acetylates histone proteins, opens up the chromatin, and allows the transcription machinery to act on the promoter for active gene transcription. In the absence of ligands (—RA), the apo-receptor pair binds to the RARE and recruits corepressor that encodes histone deacetylase (HDAC) activity, inducing histone deacetylation, chromatin condensation, and gene silencing.

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