Acts as a chaperone for the linker histone to facilitate deposition of histone B4 onto linker DNA. Required for both remodeling of sperm chromatin into nucleosomes, and linker histone binding to nucleosome core dimers. Plays a role in tissue-specific gene regulation. Required for primitive hemopoiesis, acting upstream of tal1/scl (By similarity). Forms homomultimers. Interacts with histone B4. Interacts with the B-type cyclins ccnb1 and ccnb2 (By similarity). Cytoplasmic prior to the midblastula transition, becoming predominantly nuclear subsequently. The acidic domains are probably involved in the interaction with histones. Phosphorylated by cyclin B-cdc2 kinase complexes. Belongs to the nucleosome assembly protein (NAP) family.