|Figure 2: Impact of histone modifications on transcriptional processes. The figure presents
some examples of how post-translational modifications to histones influence transcription.
(A) Acetylation of histone tails promotes an open chromatin configuration by neutralizing
their positive charge and repelling interactions with the negatively-charged DNA backbone.
The process is catalyzed by histone acetyltransferases (HATs) and reversed by histone
deacetylases (HDACs). (B) Histone modifications recruit chromatin “readers”. In this
example, the dual bromodomain protein Brd4 binds directly to acetylated histones and
recruits the elongation factor pTEF-b to stimulate (+++) release of paused polymerases
(pol II). (C) Histone modifications as indicators of the transcriptional status of
chromatin. Enhancers (Enh), gene-proximal promoters, and repressed genes (red) are
indicated by distinct patterns of histone modifications. In this case, just one example
of each type of modification is given. “H3K27ac” refers to acetylation of lysine 27
of histone H3. “H3K4me3” refers to trimethylation of lysine 4 of histone H3. “H3K27me3”
refers to trimethylation of lysine 27 of histone H3.