Stage-specific distribution of the spermatid-specific histone 2B in the rat testis

Male germ cells contain a number of histone variants, most of which are synthesized during either the mitotic or the meiotic stages of spermatogenesis. A spermatid-specific H2B (ssH2B) variant has been identified in mouse round spermatids that has an additional 12 amino acids at its carboxyl terminus as compared to somatic H2Bs. Until now, the presence of this protein in other mammals has not been known. Northern blot analysis using an oligonucleotide probe complementary to a unique coding region of the C-terminus of mouse ssH2B showed that mRNA encoding this protein was present in isolated rat round spermatids. Furthermore, immunoblot analysis of basic nuclear proteins from round spermatids probed with an anti-ssH2B antiserum that recognizes the novel peptide sequence indicated that the protein was present in round spermatids but not in pachytene spermatocytes. The ssH2B comigrated with H2A.X by acid-urea (2.5 M) PAGE and migrated slightly more slowly than TH2B by acid-urea-Triton PAGE. When seminiferous tubules were microdissected stage-specifically and basic nuclear proteins were immunoblotted, ssH2B was detected in tubule sections of stages II-VI. It reached a maximum level in stages VII-VIII and then decreased to minimal levels in stages XIII-I. Since ssH2B was not detected in pachytene spermatocytes, the stage-specific levels of ssH2B corresponded to the respective steps of spermatids present in the tubules. While ssH2B constituted a relatively small amount (approximately 2%) of total H2B protein in round spermatids, its presence in both the mouse and the rat suggests that its function may be conserved in mammalian spermatogenic cells.