Regulation of the Drosophila Enhancer of split and invected-engrailed Gene Complexes by Sister Chromatid Cohesion Proteins (original) (raw)
Figure 8
Speculative model for regulation of gene complexes by cohesin.
The top depicts a PcG-silenced complex contained in a loop created by PRE-PRE interactions. There is little or no transcription, and we posit that the silenced chromatin diameter prevents encirclement by cohesin. The nucleosomes have trimethylation of histone H3 on lysine 27 (green). The middle diagram depicts a gene complex in which cohesin, trithorax group (trxG), transcriptional activators, and PcG proteins combine to create an intermediate chromatin structure with aspects of both silenced and active regions that permits modest transcription (angled arrows); nucleosomes near the transcription start sites also have trimethylation of histone H3 on lysine 4 (pink). Based on the biphasic effects of Nipped-B and cohesin knockdown on some E(spl)-C transcripts, we posit that when cohesin levels are reduced, the chromatin structure first becomes closer to the silenced state, decreasing transcription, and that the higher order structure associated with silencing is eventually lost, leading to unrestrained transcription (bottom).