Binds to core histones and functions in the ATP-facilitated assembly of approximately regularly spaced nucleosomal arrays. May participate in parallel with other histone-binding proteins such as NAP-1. Inactive for chromatin assembly. In vitro it appears to form a high molecular mass aggregate with the core histones. Decamer formed by two pentameric rings associated in a head-to-head fashion. Expressed both maternally and zygotically, present throughout development. Highest levels are found during oogenesis and in early embryos. Belongs to the nucleoplasmin family.