Epigenetics

This is a model for the general structure (after chromatin remodeling) of the Promoter region of a gene or chromosome region that has been silenced epigenetically.

As with active transcription, the chromatin in the area of an inactive gene has usually undergone several modifications. The figure below shows the general situation for such a gene and is a good model for what the chromatin looks like in the region of an inactive gene or silenced region of the chromosome (such as heterochromatin). The DNA is methylated and Histone Methyltransferases (HMTs) have methylated specific histones. This causes a cis-effect in which the chromatin structure is condensed so that the DNA is not accessible to Transcription Factors and transcription initiation cannot occur. Additional proteins are bound: often we find HDACs which maintain a deacetylated state, and HMTs to maintain a methylated state. The proteins MeCP2 (discussed above) and HP1 are often found associated with this structure.

Common elements of a silent chromosome region:

• Methylation of H3K9 (H3K9me2), H3K27 and H4K20.
• Trimethylation: Long term silencing is associated with H3K27me3.
• Binding of heterochromatin protein 1 (HP1).
• DNA Methylation.