RESUMO
A close relationship between histone synthesis and DNA replication has been suggested for many biological systems. The presence of histonic proteins has been demonstrated in practically all kinds of eukaryotic cells. During the clevage period of sea urchins it has been postulated that histones are not present in nuclei, and that only at blastula stage they do associate with DNA. Our results suggest that the failure to isolate histonic proteins from nuclei derived from cleavage cells might be caused by the presence in those cells of proteases activated by NaHSO3. This compound has being widely used to inhibit proteolytic action in other biological systems. By changing the method for chromatin isolation, we have been able to isolate basic proteins from nuclei of gametes, zygotes and 2-4 blastomeres. These proteins behave like calf thymus histones in urea-acetic acid polyacrylamide gels, and they do not show great differences from proteins isolated from nuclei of blastula, gastrula, prism, and pluteus. The electrophoretic patterns of basic proteins obtained from eggs and zygotes are practically identical, except for one protein moving like lysine-rich calf thymus histones. This protein appears in zygote nuclei at the beginning of the first replication wave.