Structure of sea urchin sperm chromatin core particle.

Sea urchin sperm chromatin contains forms of H1, H2A, and H2B which differ from those present in adult tissues. We have delineated some effects of the variant H2A and H2B on chromatin by study of the structure of the core particle from Strongylocentrotus purpuratus sperm. The particle contains 145 base pairs of DNA and equal amounts of the four smaller histones. It sediments at 11 S and has a circular dichroism spectrum similar to that of particles containing more typical histones. The sperm core particle undergoes a shape change at low ionic strength, as observed for chicken erythrocyte particles. In contrast to these similarities, the melting profile of the sperm particle is quite different from that of erythrocyte; both the reversible transition and the irreversible denaturation of the core particle occur at higher temperatures. The sperm core particle is digested by DNase I more slowly than the core particle from chicken erythrocyte. Cutting site maps for the sperm core particle reveal the same basic organization of DNA by these histones; however, certain features of the map differ significantly from that for chicken erythrocyte, demonstrating a modulation of the canonical core particle structure by the unusual histones present in sea urchin sperm.