[Structural characteristics of lysine-rich histone from the sperm of a mollusk Anodonta piscinalis]

Sperm of freshwater bivalve mollusk Anodonta piscinalis was found to contain two fractions of lysine-rich histone: somatic histone H1 and sperm-specific protamine-like histone, named Hp. A detailed analysis of H1 and Hp structure was carried out by means of N-bromosuccinimide, chymotrypsin and pepsin cleavage followed by determination of the lysine residue number, positive charge and molecular length of obtained fragments by the method of incomplete succinylation. It has been shown, that Anodonta histone H1, like the avian histone H5, contains 3 tyrosine residues in the central hydrophobic domain of the molecule. Histone Hp contains 5 tyrosine residues, 3 of which are localized in the hydrophobic domain, while the rest two--in the COOH-terminal part of the molecule, characterized by a strong positive charge. Such unusual disposition of tyrosine residues in the lysine-rich histone has been found for the first time. All the regions of histone Hp molecule contain a great number of arginine residues. The only phenylalanine residue is localised approximately in the middle of the polypeptide chain for both H1 and Hp molecules. On the basis of structure homology between histones H1 and Hp the origin of Hp from H1 in the course of evolution is proposed.