Activators Recruit Nucleosome Modifiers (Summary)
1. In addition to direct recruitment of the transcriptional machinery, recruitment of nucleosome
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modifiers can help activate a gene packaged within chromatin
Nucleosome modifiers come in two types:
a. those that add chemical groups to the tails of histones, such as histone
acetyltransferases (HATs), which add acetyl groups
b. those that displace (or “remodel”) the nucleosomes, such as the ATP-dependent activity
of SWI/SNF
Two basic models explain how changes in nucleosomes can help the transcriptional machinery
bind at the promoter
First, remodeling, and certain modifications, can uncover DNA-binding sites that would
otherwise remain inaccessible within the nucleosome
Similarly, the addition of acetyl groups to histone tails alters the interactions between those tails
and adjacent nucleosomes. This modification is often said to “loosen” chromatin structure,
freeing up sites
A gene bearing acetylated nucleosomes at its promoter will likely have a higher affinity for the
transcriptional machinery than one with unacetylated nucleosomes
Some components of the transcriptional machinery are more stringently required at some genes
than at others, and the same applies to nucleosome modifiers as well
Thus, although all genes absolutely require RNA polymerase itself, a given gene may depend on
another particular component of the transcriptional machinery, or a nucleosome modifier, or it
may not