The chromatin of sea urchin sperm

Digestion of sea urchin sperm nuclei with micrococcal nuclease yields nucleosomal monomer fragments of 151 and 164 base pairs. Prior trypsin treatment of the sperm chromatin does not alter the size of these monomer DNA fragments despite the fact that the H1 histone is reduced to a limit globular peptide of about 83 residues. Heterologous reconstitution experiments show that this peptide is capable of protecting an extra 22 base pairs beyond the core particle in a chromatosome. Nuclease digestion of reconstitutes from DNA and sperm core histones yields a core monomer of about 141 base pairs. It is concluded that this sperm chromatin contains a chromatosome of 164 bp essentially similar to that observed in the more usual chromatins. Edman degradation of the H1 limit peptide shows its sequence to be closely analogous to the corresponding peptide of calf H1 and chicken H5. Circular dichroism studies of histone H1 from the sperm of three sea urchin species demonstrate the presence of trypsin-sensitive helical regions outside the globular domain that are absent in calf H1 and chicken H5.