CRP1, a Drosophila nuclear protein that can catalyze decondensation of demembranated Xenopus sperm chromatin was cloned and its primary structure was deduced from cDNA sequence, Alignment of deduced amino acid sequence with published sequences of other proteins revealed strong homologies to Xenopus nucleoplasmin and NO38, CRP1 is encoded by one or several closely related genes found at a single locus, position 99A on the right arm of chromosome 3, CRP1 mRNA is expressed throughout Drosophila development; it is highest during oogenesis and early embryogenesis. mRNA levels correlate closely with levels of protein expression measured previously, Results of chemical crosslinking indicate that CRP1 is either tetrameric off pentameric; similar ambiguity was revealed by direct visualization using scanning transmission electron microscopy. Consistent with previously published results, parallel crosslinking studies of Xenopus nucleoplasmin suggested a pentameric structure. Scanning transmission electron microscopic examination after negative staining revealed that CRP1 and Xenopus nucleoplasmin are morphologically similar. CRP1 is able to substitute for nucleoplasmin in Xenopus egg extract-mediated sperm chromatin decondensation. In vitro, CRP1-induced decondensation is accompanied by direct binding of CRP1 to chromatin. (C) 1997 Academic Press.