Histone H2A subtypes associate interchangeably in vivo with histone H2B subtypes.

The yeast Saccharomyces cerevisiae contains two primary sequence subtypes of histone H2B (H2B1 and H2B2) and of H2A (H2A1 and H2A2). Mutants in each of the H2B subtypes have been used to show previously that yeast cells lacking one or the other, but not both, of the H2B proteins are viable. Because H2A protein interacts in the nucleosome with H2B, we wished to determine whether specific H2A subtypes must interact with specific H2B subtypes. We describe experiments in which frameshift mutations were introduced into both of the H2A genes in vitro and the mutant genes integrated into the yeast genome, replacing the wild-type H2A genes by a subsequent recombination. Using these mutant (hta1- and hta2-) strains we find that neither H2A gene has a unique essential function during any phase of the yeast life cycle, although strains homozygous for hta1- grow more slowly. However, one functional H2A gene is required for viability because cells mutant in both H2A genes arrest at spore germination prior to bud separation. By combining these H2A mutations with the H2B mutations obtained previously, we show that all combinations of H2A and H2B subtypes produce viable cells. From these genetic experiments and electrophoretic analysis of the histone proteins of these mutants we conclude that the H2A subtypes can associate interchangeably with the H2B subtypes.