Histone H3.3C
euchromatin
H3.5 is a novel hominid-specific histone H3 variant that is specifically expressed in the seminiferous tubules of human testes.
The incorporation of histone variants into chromatin plays an important role for the establishment of particular chromatin states. Six human histone H3 variants are known to date, not counting CenH3 variants: H3.1, H3.2, H3.3 and the testis-specific H3.1t as well as the recently described variants H3.X and H3.Y. We report the discovery of H3.5, a novel non-CenH3 histone H3 variant. H3.5 is encoded on human chromosome 12p11.21 and probably evolved in a common ancestor of all recent great apes (Hominidae) as a consequence of H3F3B gene duplication by retrotransposition. H3.5 mRNA is specifically expressed in seminiferous tubules of human testis. Interestingly, H3.5 has two exact copies of ARKST motifs adjacent to lysine-9 or lysine-27, and lysine-79 is replaced by asparagine. In the Hek293 cell line, ectopically expressed H3.5 is assembled into chromatin and targeted by PTM. H3.5 preferentially colocalizes with euchromatin, and it is associated with actively transcribed genes and can replace an essential function of RNAi-depleted H3.3 in cell growth.