Epigenetics

This is a model for the general structure (after chromatin remodeling) of the Promoter region of an actively transcribed gene.

Usually, the chromatin in the area of an actively expressed gene has undergone several modifications and so is remodeled in each of the ways shown above. 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 actively transcribed gene. A modification has occurred, such as histone acetylation. This causes a cis-effect in which the chromatin structure is opened up and Transcription Factors can bind. Readers have also bound and caused histone exchange so that there are H3.3 histones in nucleosomes at the TSS (Transcription Start Site). Other Readers have induced further modification so that nucleosomes near the TSS have H3K4me3, a common signature of active transcription.

Common elements of an actively transcribed gene:

• Histone Acetylation, particularly H4K16ac and/or H4K14ac, and open chromatin around the promoter as a result. The DNA around the promoter is relatively free of nucleosomes.
• Trimethylation: H3K4me3 at nucleosomes near the TSS is a mark of active transcription.
• Nucleosomes just downstream from the TSS have the histone variant H3.3 instead of the variants H3.1 and H3.2 that are found elsewhere.