A Bridging Model for Persistence of a Polycomb Group Protein Complex through DNA Replication In Vitro

Abstract

SUMMARY Epigenetic regulation may involve heritable chromatin states but how chromatin features can be inherited through DNA replication is incompletely understood. We address this question using cell free replication of chromatin. Previously, we showed that a Polycomb Group complex, PRC1, remains continuously associated with chromatin through DNA replication. Here we investigate the mechanism of persistence. We find that a single PRC1 subunit, Posterior Sex Combs (PSC) can reconstitute persistence through DNA replication. PSC binds nucleosomes and self-interacts, bridging nucleosomes into a stable, oligomeric structure. Within these structures, individual PSC-chromatin contacts are dynamic. Stable association of PSC with chromatin, including through DNA replication, depends on PSC-PSC interactions. Our data suggest labile individual PSC-chromatin contacts allow passage of the DNA replication machinery while PSC-PSC interactions prevent PSC from dissociating, allowing it to rebind to replicated chromatin. This mechanism may allow inheritance of chromatin proteins including PRC1 through DNA replication to maintain chromatin states.

Epigenetic inheritance: histone bookmarks across generations

“…At first, this finding with PRC1 seems reminiscent of that with the SV40 replication machinery, which can traverse cross-linked histone octamers without histone eviction, leaving nucleosomes intact [57], however, the mechanism by which PRC1 remains associated with replicated DNA is quite different. Psc self-associates and is capable of bridging nucleosomal templates [58]. This has led to the proposal of a mechanism of epigenetic inheritance by which PRC1 complexes transiently associate with multiple contacts ahead and behind the replication fork (figure 1).…”

Polycomb repression in the regulation of growth and development in Arabidopsis

“…In addition, PcG proteins compact the chromatin template by promoting inter-nucleosomal interactions (PRC1), repress transcriptional activity (PRC1), and organize chromatin domains in the nucleus such as clustering of PcG targets within Pc foci and forming topologically associating domains (PRC1, PRC2) [4][5][6][7][8][9][10][11]. Of note, the repressive chromatin state is passed on from mother to daughter cells; current evidence suggests PcG proteins associated with the replication fork copy the repressed state onto the newly synthesized chromatin [12,13]. A suppressor screen of Polycomb mutant in Drosophila identified the Trithorax Group (TrxG) proteins as acting antagonistically to PcG proteins.…”

Asymmetric distribution of histones during Drosophila male germline stem cell asymmetric divisions

“…Interestingly, PcG remains at the DNA replication forks in vitro (Francis, 2009b, a, Francis et al, 2009). Interactions between two different PcG complexes through dimerization can prevent their dissociation from chromatin during DNA replication (Lengsfeld et al, 2012, Lo et al, 2012). A recent in vivo study examined histone methylation marks in S phase cells from Drosophila embryos (Petruk et al, 2012).…”

“…For example, the H3K27me3 mark generated by PcG could be recognized by the chromodomain of Polycomb protein. Because Polycomb group proteins can dimerize (Min et al, 2003, Lengsfeld et al, 2012, Lo et al, 2012) and directly interact with PCNA at the replication fork (Petruk et al, 2012), it is possible that, through dimerization, Polycomb coordinates preferential retention of preexisting H3-H4 tetramers carrying the H3K27me3 mark onto one sister chromatid at the replication fork (Figure 2B). Epigenetic marks on preexisting histones may contribute to maintain unique gene expression pattern in stem cells.…”