Modification of Histones during Spermiogenesis in Trout: A Molecular Mechanism for Altering Histone Binding to DNA

At a late stage of spermatogenesis in rainbow-trout testis, the entire complement of histones is replaced by newly synthesized protamine and histones are extensively phosphorylated and acetylated. Tryptic digestion of purified histones labeled by incubation of testicular cells with [32P]phosphate shows that phosphorylation occurs at a small number of seryl residues. Histone I (lysine-rich) is phosphorylated in the sequence Lys-Ser(PO4)-Pro-Lys, which is located in the lysine-rich C-terminal region of the molecule. Histones IIb1(slightly lysine-rich) and IV (glycine, arginine-rich) give rise to the same phosphopeptide, Ac-Ser(PO4)-Gly-Arg, which comprises the amino terminus of each histone. Thermolysin digests of phosphohistones IIb1and IV also released a phosphopeptide with composition corresponding to the first six residues of histone IV: Ac-Ser(PO4)-Gly-Arg-Gly-Lys-Gly. An α -helical model of the N-terminal region of histone IV shows that this region is a possible DNA-binding site. Phosphorylation at serine 1 together with ε -amino acetylation at lysines 5, 8, 12, and 16 (observed in histone IV from trout testis) could profoundly modify ionic interactions and lead to an ``unzipping'' of histone IV from DNA