The N-terminus of histone H2B, but not that of histone H3 or its phosphorylation, is essential for chromosome condensation

We have studied the role of individual histone N-termini and the phosphoryl ation of histone H3 in chromosome condensation. Nucleosomes, reconstituted with histone octamers containing different combinations of recombinant full -length and tailless histones, were used as competitors for chromosome asse mbly in Xenopus egg extracts. Nucleosomes reconstituted with intact octamer s inhibited chromosome condensation as efficiently as the native ones, whil e tailless nucleosomes were unable to affect this process. Importantly, the addition to the extract of particles containing only intact histone H2B st rongly interfered with chromosome formation while such an effect was not ob served with particles lacking the N-terminal tail of H2B. This demonstrates that the inhibition effect observed in the presence of competitor nucleoso mes is mainly due to the N-terminus of this histone, which, therefore, is e ssential for chromosome condensation. Nucleosomes in which all histones but H3 were tailless did not impede chromosome formation. In addition, when co mpetitor nucleosome particles were reconstituted with full-length H2A, H2B and H4 and histone H3 mutated at the phosphorylable serine 10 or serine 28, their inhibiting efficiency was identical to that of the native particles. Hence, the tail of H3, whether intact or phosphorylated, is not important for chromosome condensation. A novel hypothesis, termed 'the ready producti on label' was suggested to explain the role of histone H3 phosphorylation d uring cell division.