HIRA is critical for a nucleosome assembly pathway independent of DNA synthesis

The mammalian HIRA gene encodes a histone-interacting protein whose homolog in Xenopus laevis is characterized here. In vitro, recombinant Xenopus HIRA bound purified core histones and promoted their deposition onto plasmid DNA. The Xenopus HIRA protein, tightly associated with nuclear structures in somatic cells, was found in a soluble maternal pool in early embryos. Xenopus egg extracts, known for their chromatin assembly efficiency, were specifically immunodepleted for HIRA. These depleted extracts were severely impaired in their ability to assemble nucleosomes on nonreplicated DNA, although nucleosome formation associated with DNA synthesis remained efficient. Furthermore, this defect was largely corrected by reintroduction of HIRA along with (H3-H4)(2) tetramers. We thus delineate a nucleosome assembly pathway that depends on HIRA.