No abstract

Summary Histone variants play crucial roles in gene expression, genome integrity and chromosome segregation. However, to what extent histone variants control chromatin architecture remains largely unknown. We report genome-wide profiles of all four types of H2A variants in Arabidopsis and identify that the previously uncharacterized histone variant H2A.W specifically associates with heterochromatin. Genetic analyses show that H2A.W acts in synergy with the heterochromatic marks H3K9me2 and DNA methylation to maintain genome integrity. In vitro, H2A.W enhances chromatin condensation through a higher propensity to promote fiber-to-fiber interactions via its conserved C-terminal motif. In vivo, elimination of H2A.W causes decondensation of heterochromatin and conversely, ectopic expression of H2A.W promotes heterochromatin condensation. These results demonstrate that H2A.W plays critical roles in heterochromatin by promoting higher order chromatin condensation. Since motifs similar to the H2A.W C-terminal motif are present in other histone variants in other organisms, our findings impact our understanding of heterochromatin condensation in eukaryotes.

Higher order folding of chromatin fibre is mediated by interactions of the histone H4 N-terminal tail domains with neighbouring nucleosomes. Mechanistically, the H4 tails of one nucleosome bind to the acidic patch region on the surface of adjacent nucleosomes, causing fibre compaction. The functionality of the chromatin fibre can be modified by proteins that interact with the nucleosome. The co-structures of five different proteins with the nucleosome (LANA, IL-33, RCC1, Sir3 and HMGN2) recently have been examined by experimental and computational studies. Interestingly, each of these proteins displays steric, ionic and hydrogen bond complementarity with the acidic patch, and therefore will compete with each other for binding to the nucleosome. We first review the molecular details of each interface, focusing on the key non-covalent interactions that stabilize the protein -acidic patch interactions. We then propose a model in which binding of proteins to the nucleosome disrupts interaction of the H4 tail domains with the acidic patch, preventing the intrinsic chromatin folding pathway and leading to assembly of alternative higher order chromatin structures with unique biological functions.

Background: The histone chaperone FACT binds and reorganizes nucleosomes during critical cellular processes.Results: FACT binds histones, DNA, and mono- and tri-nucleosomes with high affinity. FACT reduces non-nucleosomal histone/DNA interactions.Conclusion: Multiple regions of FACT strategically bind target sites on nucleosomes to coordinate (dis)assembly.Significance: The thermodynamic parameters guiding multiple FACT/nucleosome interaction(s) coincide with reorganization events.