Understanding the molecular mechanisms regulating chromosome AC220 mw architecture will thus be crucial in future research in this field. A general rule emerges from 3C-based approaches: topological domains associated to open chromatin establish long range contacts with other active domains, whereas repressed chromatin regions tend to cluster together (Figure 2) [18, 22 and 23]. In particular, this has been well documented for H3K27me3 associated

chromatin. For example, FISH studies show that the Drosophila Antp and Abd-B genes, which are separated by 10 Mb and located in the ANT-C and BX-C Hox clusters, co-localize inside a PC focus when repressed, but not when any of them is active. 4C analysis confirms this contact and shows that the BX-C locus can establish several other interactions, mainly with other H3K27me3 genomic domains located on the same chromosome [ 12•]. Importantly, long-range interactions have been reported for transgenes containing regulatory regions of the BX-C including PREs associated with insulator activity, such as Fab7 and Mcp [ 43 and 44], whereas another PRE devoid of insulators,

bxd, does not induce long-range contacts. In keeping with these data, the insulator selleck kinase inhibitor portion of the Mcp and Fab7 are required and sufficient to establish long-range interactions [ 45 and 46], suggesting that PcG proteins may stabilize long-range interactions rather than induce them. On the other side of the coin, contacts can also occur when both target genes are active. Those contacts are functionally regulated because they rely on Trithorax, enhancer specificity and CTCF proteins [ 12• and 46]. These 5-Fluoracil order studies thus confirm the segregation between active open chromatin and repressed compact chromatin, because a high frequency of interactions is never observed between active and silenced genes. The same theme emerges from several recent studies that analyzed long-range interactions

in pluripotent stem cells and found significant co-localization of chromatin regions characterized by high pluripotency factor occupancy in mammals [47•, 48•, 49•, 50•, 51• and 52•]. Once again, long-range contacts involved either active genes or silent chromatin, where many long-range interactions involve domains enriched in Polycomb/H3K27me3 in embryonic stem cells. Importantly, loss of the protein Polycomb Eed decreases contacts between Polycomb-regulated regions without altering the overall chromosome conformation [52•]. Long-range interaction of H3K27me3 chromatin domains has also been reported during vernalization in Arabidopsis, when cold induces silencing of the flowering locus c (FLC). Live cell imaging shows that FLC alleles, tagged with the Lac operator system, cluster during cold.