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. 2013 Jun 13;41(15):7185–7199. doi: 10.1093/nar/gkt499

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© The Author(s) 2013. Published by Oxford University Press.

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.

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Figure 3. — Chromatin looping is responsible for forming higher-order hub-like structures within the nucleus. (a) An active chromatin hub (ACH) is a structure that allows enhancers and promoters to come into close spatial proximity with each other (40). This structure has a high concentration of chromatin remodellers and PolII, which allows stable/high levels of transcription. Interactions between promoters and enhancers and represented by dashed lines. (b) The recruitment of genes and enhancers to a repressive chromatin hub (RCH) results in their downregulation (165). This structure potentially prevents enhancers from communicating with their cognate promoter by looping them out and preventing them from interacting. This structure may also restrict the amount of PolII from binding to gene promoters. (c and d) During development, genes in the Hox locus show a linear movement from a repressive chromatin structure to a region where they are expressed (166). This movement allows enhancers to interact with targets that were previously held in the repressive domain.