Histone variants and chromatin structure during sea urchin development

At the late blastula stage of sea urchin development a changeover of histone synthesis and chromatin composition takes place. Synthesis of the early histone variants declines while another set, the late histone variants, begins to be detected. During subsequent development the late histones accumulate steadily. In the 9-day larva only late histone variants are detectable. Micrococcal nuclease acts differentially on early and late nuclei. There is a depressed release of acid-soluble DNA when chromatin containing the late histones is digested. Nucleosomal repeat lengths change systematically and in parallel with the changing histone composition. Blastula and preblastula chromatin have a significantly shorter major repeat length than does the chromatin of 9-, 11-, and 16-day larvae. Intermediate stages of development have chromatin with intermediate periodicities. These differences are observed when the determinations are made under denaturing conditions of electrophoresis. Repeat lengths were found to be independent of the extent of digestion at all stages examined except the pluteus, in which there is an increase of the apparent repeat length as digestion proceeds. Pancreatic DNase I digests nuclei from blastulae and 9-day larvae similarly. Changes in the histone composition of chromatin, in nuclease accessibility of chromatin, and in nucleosomal repeat length are all very closely correlated, implying that there are underlying causal relationships.