An Epigenetic Switch Involving Overlapping Fur and DNA Methylation Optimizes Expression of a Type VI Secretion Gene Cluster

Type VI secretion systems (T6SS) are macromolecular machines of the cell envelope of Gram-negative bacteria responsible for bacterial killing and/or virulence towards different host cells. Here, we characterized the regulatory mechanism underlying expression of the enteroagregative Escherichia coli sci1 T6SS gene cluster. We identified Fur as the main regulator of the sci1 cluster. A detailed analysis of the promoter region showed the presence of three GATC motifs, which are target of the DNA adenine methylase Dam. Using a combination of reporter fusion, gel shift, and in vivo and in vitro Dam methylation assays, we dissected the regulatory role of Fur and Dam-dependent methylation. We showed that the sci1 gene cluster expression is under the control of an epigenetic switch depending on methylation: fur binding prevents methylation of a GATC motif, whereas methylation at this specific site decreases the affinity of Fur for its binding box. A model is proposed in which the sci1 promoter is regulated by iron availability, adenine methylation, and DNA replication

Heterochromatin and the molecular mechanisms of 'parent-of-origin' effects in animals.

Twenty five years ago it was proposed that conserved components of constitutive heterochromatin assemble heterochromatinlike complexes in euchromatin and this could provide a general mechanism for regulating heritable (cell-to-cell) changes in gene expressibility. As a special case, differences in the assembly of heterochromatin-like complexes on homologous chromosomes might also regulate the parent-of-origin-dependent gene expression observed in placental mammals. Here, the progress made in the intervening period with emphasis on the role of heterochromatin and heterochromatin-like complexes in parent-of-origin effects in animals is reviewed

Coupling mitosis to DNA replication: The emerging role of the histone H4-lysine 20 methyltransferase PR-Set7

To ensure accurate inheritance of genetic information through cell proliferation, chromosomes must be precisely copied only during S phase, and then correctly condensed and segregated during mitosis. Several new findings suggest that this tight coupling between DNA replication and mitosis is in part controlled by cell cycle regulated chromatin modifications, in particular due to the changing activity of lysine methyltransferase PR-Set7/SET8 that is responsible for the monomethylation of histone H4 at lysine 20. Cell cycle oscillation of PR-Set7 is orchestrated by ubiquitin-mediated proteolysis, and interference with this regulatory process leads to unscheduled licensing of replication origins and altered timing of mitotic chromosome compaction. This review provides an overview of how PR-Set7 regulates these two cell cycle events and highlights questions that remain to be addressed

Coupling mitosis to DNA replication: The emerging role of the histone H4-lysine 20 methyltransferase PR-Set7

To ensure accurate inheritance of genetic information through cell proliferation, chromosomes must be precisely copied only during S phase, and then correctly condensed and segregated during mitosis. Several new findings suggest that this tight coupling between DNA replication and mitosis is in part controlled by cell cycle regulated chromatin modifications, in particular due to the changing activity of lysine methyltransferase PR-Set7/SET8 that is responsible for the monomethylation of histone H4 at lysine 20. Cell cycle oscillation of PR-Set7 is orchestrated by ubiquitin-mediated proteolysis, and interference with this regulatory process leads to unscheduled licensing of replication origins and altered timing of mitotic chromosome compaction. This review provides an overview of how PR-Set7 regulates these two cell cycle events and highlights questions that remain to be addressed

The histone H4 Lys 20 methyltransferase PR-Set7 regulates replication origins in mammalian cells

The initiation of DNA synthesis is governed by the licensing of replication origins, which consists of assembling a pre-replication complex (pre-RC) on origins during late M- and G1-phases. In metazoans, functional replication origins do not show defined DNA consensus sequences, thus evoking the involvement of chromatin determinants in the selection of these origins. Here, we show that the onset of licensing in mammalian cells coincides with an increase in histone H4 Lys 20 monomethylation (H4K20me1) at replication origins by the methyltransferase PR-Set7 (also known as Set8 or KMT5A). Indeed, tethering PR-Set7 methylase activity to a specific genomic locus promotes the loading of pre-RC proteins on chromatin. In addition, we demonstrate that PR-Set7 undergoes a PCNA- and Cul4-Ddb1-driven degradation during S phase that contributes to the disappearance of H4K20me1 at origins and the inhibition of replication licensing. Strikingly, expression of a PR-Set7 mutant insensitive to this degradation causes the maintenance of H4K20me1 and repeated DNA replication at origins. These results elucidate a critical role for PR-Set7 and H4K20me1 in the chromatin events that regulate replication origins

SUMO ylated PRC 1 controls histone H3.3 deposition and genome integrity of embryonic heterochromatin

Chromatin integrity is essential for cellular homeostasis. Polycombgroup proteins modulate chromatin states and transcriptionallyrepress developmental genes to maintain cell identity. They alsorepress repetitive sequences such as major satellites and consti-tute an alternative state of pericentromeric constitutive hete-rochromatin at paternal chromosomes (pat-PCH) in mouse pre-implantation embryos. Remarkably, pat-PCH contains the histoneH3.3 variant, which is absent from canonical PCH at maternal chro-mosomes, which is marked by histone H3 lysine 9 trimethylation(H3K9me3), HP1, and ATRX proteins. Here, we show that SUMO2-modified CBX2-containing Polycomb Repressive Complex 1 (PRC1)recruits the H3.3-specific chaperone DAXX to pat-PCH, enablingH3.3 incorporation at these loci. Deficiency of Daxx or PRC1 compo-nents Ring1 and Rnf2 abrogates H3.3 incorporation, induces chro-matin decompaction and breakage at PCH of exclusively paternalchromosomes, and causes their mis-segregation. Complementationassays show that DAXX-mediated H3.3 deposition is required forchromosome stability in early embryos. DAXX also regulates repres-sion of PRC1 target genes during oogenesis and early embryogene-sis. The study identifies a novel critical role for Polycomb inensuring heterochromatin integrity and chromosome stability inmouse early development