Two highly specialized histone H1 proteins are the major chromosomal proteins of the sperm of the sea anemone Urticina (Tealia) crassicornis.

The sperm nuclear basic proteins (SNBPs) from a sea anemone (Urticina crassicornis) have been isolated and characterized for the first time. They consist of two sperm-specific members of the histone H1 family with Mr 22,700 and 24,600. They amount to about 60--70% of the total chromosomal sperm proteins. Their amino acid composition and the primary structure of their trypsin-resistant core indicate a strong relation to histone H5 from the nucleated erythrocytes of birds and amphibians as well as to other high sperm-specific H1-like (PL-I) proteins from phylogenetically distant groups. The major presence of histone H1-like protein in the sperm of an organism belonging to such a low phylogenetic group provides experimental support to the hypothesis that SNBPs may all have evolved from a primitive histone precursor.