Trypanosome histone variants H3.V and H4.V promote nucleosome plasticity in repressed chromatin.

Abstract

Histone variants define distinct chromatin states by modulating the biophysical properties of nucleosomes. Variants play a particularly important role in the parasitic protist Trypanosoma brucei, which has unusual chromatin and lacks a canonical repressive heterochromatin system. Instead, T. brucei utilises specialised divergent histone variants H3.V and H4.V. However, the biochemical basis of their repressive functions is unknown. Here, we determined the structure of the H3.V-H4.V nucleosome core particle and biochemically characterised variant-containing nucleosomes and nucleosome arrays probing their unique properties. We discovered that surprisingly for repressive-state nucleosomes, H3.V promotes pronounced DNA splaying, largely via its N-terminal tail region, whilst retaining overall stability that is comparable to canonical nucleosomes. In contrast, H4.V exhibits near-identical binding to DNA but mediates a slight increase in histone octamer stability. The surface of the H3.V-H4.V nucleosome is altered and provides a differential platform for chromatin binding proteins, linking the variants to parasite pathogenicity.