Transcriptional read-through is not sufficient to induce an epigenetic switch in the silencing activity of Polycomb response elements

Abstract

In Drosophila, Polycomb (PcG) and Trithorax (TrxG) group proteins are assembled on Polycomb response elements (PREs) to maintain tissue and stage-specific patterns of gene expression. Critical to coordinating gene expression with the process of differentiation, the activity of PREs can be switched "on" and "off." When on, the PRE imposes a silenced state on the genes in the same domain that is stably inherited through multiple rounds of cell division. When the PRE is switched off, the domain is in a state permissive for gene expression that can be stably inherited. Previous studies have suggested that a burst of transcription through a PRE sequence displaces PcG proteins and provides a universal mechanism for inducing a heritable switch in PRE activity from on to off; however, the evidence favoring this model is indirect. Here, we have directly tested the transcriptional read-through mechanism. Contrary to previous suggestions, we show that transcription through the PRE is not sufficient for inducing an epigenetic switch in PRE activity. In fact, even high levels of continuous transcription through a PRE fails to dislodge the PcG proteins, nor does it remove repressive histone marks. Our results indicate that other mechanisms involving adjacent DNA regulatory elements must be implicated in heritable switch of PRE activity.Polycomb | chromatin silencing | bithorax | intergenic transcription | Trithorax

Transcription through Polycomb response elements does not induce a switch from repression to activation

“…PcG protein complexes are bound at intronic PREs of HOX genes in cells where these genes are actively transcribed, both in developing Drosophila (10-12) and in tissue culture cells (13). Similar to the situation at the reporter gene in the Erokhin et al study (1), the level of PcG protein binding at such intronic PREs is somewhat reduced when analyzed by chromatin immunoprecipitation, suggesting that the act of transcription may to some extent interfere with PcG complex binding, but certainly does not result in complete removal of these protein complexes.…”

“…In 2005, Schmitt, Prestel, and Paro (9) reported that transcription through the same Fab-7 fragment (again 3.6 kb) caused switching from a repressor to an activator, but only if the transcription was continuous, at least through embryogenesis, and the authors proposed that PcG proteins would be knocked off by transcription through PREs. With their data, Erokhin et al (1) argue that this is not the case because even prolonged transcription through the PRE in their transgene does not result in displacement of PcG protein complexes. Does persistent transcription through PREs permanently displace Polycomb proteins in their native chromosomal context?…”

“…Repression of a target gene by PcG proteins does, however, not always mean terminal silencing of the gene in a given cell; in many instances repression needs to be reversed, even after prolonged periods of silencing. In PNAS, Erokhin et al (1) address how PcGregulated transgenes respond to the presence of a transcriptional activator. Understanding how a PcG-repressed state is converted into a transcriptionally active state is not only important for understanding how genes are regulated during normal development but also for understanding the events that occur when PcG proteins malfunction in cancer.…”

“…1. Erokhin et al (1) found that continuous Gal4 expression is able to disrupt transcriptional repression mediated by the PRE, and they demonstrate that this process does not require transcription through the PRE. Moreover, the authors show that transcription through the PRE in the transgene does not cause displacement of PcG proteins.…”

“…A last point deserving discussion comes from the observation by Erokhin et al (1) that binding of Gal4 to the transgene is able to overcome repression mediated by the PRE and restore normal levels of white gene transcription. It should be noted that in their transgene, the Gal4 protein binding sites are located 8 kb and 1.7 kb away from the white gene and the PRE, respectively, and that, at least in yeast, Gal4 is unable to activate transcription at this distance (17).…”