RYBP stimulates PRC1 to shape chromatin-based communication between Polycomb repressive complexes.

Polycomb group (PcG) proteins function as chromatin-based transcriptional repressors that are essential for normal gene regulation during development. However, how these systems function to achieve transcriptional regulation remains very poorly understood. Here, we discover that the histone H2AK119 E3 ubiquitin ligase activity of Polycomb repressive complex 1(PRC1) is defined by the composition of its catalytic subunits and is highly regulated by RYBP/YAF2-dependent stimulation. In mouse embryonic stem cells, RYBP plays a central role in shaping H2AK119 mono-ubiquitylation at PcG targets and underpins an activity-based communication between PRC1 and Polycomb repressive complex 2(PRC2) which is required for normal histone H3 lysine 27 trimethylation(H3K27me3). Without normal histone modification-dependent communication between PRC1 and PRC2, repressive Polycomb chromatin domains can erode, rendering target genes susceptible to inappropriate gene expression signals. This suggests that activity-based communication and histone modification-dependent thresholds create a localized form of epigenetic memory required for normal PcG chromatin domain function in gene regulation

Early-late genes of the ecdysone cascade as models for transcriptional studies

<p>The <i>DHR3</i> and <i>Hr4</i> early-late genes of the ecdysone cascade are described as models for transcriptional studies in <i>Drosophila</i> cells. In a set of experiments, it became clear that these genes are a convenient and versatile system for research into the physiological conditions upon 20-hydroxyecdysone induction. <i>DHR3</i> and <i>Hr4</i> gene transcription is characterized by fast activation kinetics, which enables transcriptional studies without the influence of indirect effects. A limited number of activated genes (only 73 genes are induced one hour after treatment) promote the selectivity of transcriptional studies via 20-hydroxyecdysone induction. <i>DHR3</i> and <i>Hr4</i> gene expression is dose dependent, is completely controlled by the hormone titer and decreases within hours of 20-hydroxyecdysone withdrawal. The <i>DHR3</i> and <i>Hr4</i> gene promoters become functional within 20 minutes after induction, which makes them useful tools for investigation if the early activation process. Their transcription is controlled by the RNA polymerase II pausing mechanism, which is widespread in the genome of <i>Drosophila melanogaster</i> but is still underinvestigated. Uniform expression activation of the <i>DHR3</i> and <i>Hr4</i> genes in a cell population was confirmed at both the RNA and protein levels. Homogeneity of the transcription response makes DHR3/Hr4 system valuable for investigation of the protein dynamics during transcription induction.</p