Polycomb complexes establish chromatin modifications for maintaining gene repression and are essential for embryonic development in mice. Here we use pluripotent embryonic stem (ES) cells to demonstrate an unexpected redundancy between Polycomb-repressive complex 1 (PRC1) and PRC2 during the formation of differentiated cells. ES cells lacking the function of either PRC1 or PRC2 can differentiate into cells of the three germ layers, whereas simultaneous loss of PRC1 and PRC2 abrogates differentiation. On the molecular level, the differentiation defect is caused by the derepression of a set of genes that is redundantly repressed by PRC1 and PRC2 in ES cells. Furthermore, we find that genomic repeats are Polycomb targets and show that, in the absence of Polycomb complexes, endogenous murine leukemia virus elements can mobilize. This indicates a contribution of the Polycomb group system to the defense against parasitic DNA, and a potential role of genomic repeats in Polycomb-mediated gene regulation.

Pax5 controls the identity and development of B cells by repressing lineage‐inappropriate genes and activating B‐cell‐specific genes. Here, we used genome‐wide approaches to identify Pax5 target genes in pro‐B and mature B cells. In these cell types, Pax5 bound to 40% of the cis ‐regulatory elements defined by mapping DNase I hypersensitive (DHS) sites, transcription start sites and histone modifications. Although Pax5 bound to 8000 target genes, it regulated only 4% of them in pro‐B and mature B cells by inducing enhancers at activated genes and eliminating DHS sites at repressed genes. Pax5‐regulated genes in pro‐B cells account for 23% of all expression changes occurring between common lymphoid progenitors and committed pro‐B cells, which identifies Pax5 as an important regulator of this developmental transition. Regulated Pax5 target genes minimally overlap in pro‐B and mature B cells, which reflects massive expression changes between these cell types. Hence, Pax5 controls B‐cell identity and function by regulating distinct target genes in early and late B lymphopoiesis.

Pax5 is a critical regulator of B‐cell commitment. Here, we identified direct Pax5 target genes by streptavidin‐mediated ChIP‐chip analysis of pro‐B cells expressing in vivo biotinylated Pax5. By binding to promoters and enhancers, Pax5 directly regulates the expression of multiple transcription factor, cell surface receptor and signal transducer genes. One of the newly identified enhancers was shown by transgenic analysis to confer Pax5‐dependent B‐cell‐specific activity to the Nedd9 gene controlling B‐cell trafficking. Profiling of histone modifications in Pax5‐deficient and wild‐type pro‐B cells demonstrated that Pax5 induces active chromatin at activated target genes, while eliminating active chromatin at repressed genes in committed pro‐B cells. Pax5 rapidly induces these chromatin and transcription changes by recruiting chromatin‐remodelling, histone‐modifying and basal transcription factor complexes to its target genes. These data provide novel insight into the regulatory network and epigenetic regulation, by which Pax5 controls B‐cell commitment.