hv1 is an evolutionarily conserved H2A variant that is preferentially associated with active genes

Abstract

Polyclonal antibodies to the Tetrahymena macronuclear-specific histone variant hv1 cross-react with histone-like molecules from yeast, wheat, and mouse. A novel purification scheme has allowed isolation of sufficient hv1 to enable determination of the sequence of 61 amino-terminal residues as well as 27 additional internal residues. These data clearly demonstrate that hv1 shares a number of conserved sequence elements with the H2A family of histones. Comparison of hv1 with H2A.F (= H2A.Z = M1), another evolutionarily conserved H2A variant whose sequence is known, reveals that they share an unblocked amino-terminal alanine (instead of acetylserine) and a distinctive structure in a "variant box" region that distinguishes them from major H2As. In addition, 10 residues have been identified which are identical (or highly similar) in hv1 and H2A.F, but are different from residues conserved in the major H2As. Therefore, in many ways hv1 resembles chick H2A.F more than the major Tetrahymena H2A. The sites of acetylation of hv1 also differ from those of the major Tetrahymena H2As. In spite of their similarities, hv1 and H2A.Z differ significantly in their amino termini, and antibodies against hv1 do not react with H2A.Z. Interestingly, the nucleolar staining pattern reported with anti-hv1 serum is similar to that reported for an antiserum to another H2A variant, mouse testes-enriched H2A.X. Since both H2A.Z and hv1 appear to be enriched in transcriptionally active chromatin, these results suggest that there may be a number of different, functionally distinct, nonallelic variants in the H2A family of histones and that hv1 is a hybrid H2A variant with properties of both vertebrate H2A.Z and H2A.X.