Sea urchin sperm chromatin structure as probed by pancreatic DNase I: Evidence for a novel cutting periodicity

Chromatin from mature sea urchin spermatozoa is highly compacted and composed almost entirely of DNA and the five histones, although sperm H1, H2A, and H2b histones differ from those found in embryo or somatic cell nuclei. Release of acid-soluble DNA during pancreatic DNase I digestion is 20-fold slower from sperm nuclei than from embryonic nuclei. Following DNase I digestion, most sperm nuclear DNA remains at high molecular weight, although there appears to be some release of 10 base oligomer fragments. Size analysis of the higher molecular weight DNA reveals a series of fragments that indicate a cutting periodicity of approximately 500 base pairs. This pattern remains when electrophoretic separation is carried out under denaturing conditions. The 500 base pair cleavage pattern was not detected in digestions of embryonic nuclei. Nucleosomes reconstituted with fractionated core histones from sperm gave, upon digestion, a characteristic 10 base “ladder,” with no resistant high molecular weight DNA. Micrococcal nuclease and DNase II digested sperm nuclei to produce DNA fragments with a calculated repeat length of 248 ± 3 and 246 ± 6 base pairs, respectively. The structural basis for the 500 base pair cutting periodicity in sperm nuclei may reside in the unique sperm H1 histone.