The sea urchin sperm histone H2B readily forms a complex with heterologous H2A despite having an elongated N-terminal domain.

A test has been made of the postulate that interaction between histones H2A and H2B occurs only in the C-terminal domain of H2B and is independent of the N-terminal domain in which the sites of chemical modification occur. Sea urchin sperm H2B's have much extended N-terminal domains when compared to other studied H2B's and the interactions of Sphaerechinus granularis H2B1 with homologous H2A and with calf thymus H2A have been studied. Continuous variation analysis of circular dichroism results indicates that both homologous and heterologous H2A/H2B pairs can cooperatively form complexes, which in all cases involve the same number of helical residues (approximately equal to 80). It is concluded that although the sequence of the N-terminal domain of H2B can indirectly influence the free solution association constant with histone H2A, it does not take part in the interaction.