Isolation and characterization of TH2A, a germ cell-specific variant of histone 2A in rat testis.

The histones of mammalian testis nuclei are much more complex than those of somatic tissue nuclei, and their resolution into variants can be greatly improved by polyacrylamide gel electrophoresis in the presence of Triton X-100.A previously unidentified germ cellspecific variant, THBA, has been isolated in pure form and characterized by analysis of its amino acids and tryptic peptides as a variant of H2A.The electrophoretic mobility of TH2A is similar to H2A.1 on Tritonacid-urea and sodium dodecyl sulfate gels, but more like H2B on acid-urea gels.TH2A first appears in the testis of 16-day-old rats, a time that coincides with the appearance of primary spermatocytes in the maturing testis, and with further germ cell development the level of TH2A increases.Synthesis of testis histones, as measured by tritiated amino acid incorporation into histones in vivo, was studied using highly purified pachytene spermatocytes (98%) and early spermatids (97%) obtained by elutriation and Percoll density gradient centrifugation.Negligible synthesis of any histone was detectable in the early spermatids.In con- trast, high rates of synthesis of THSA, as well as the somatic forms, H2A.1 and H2A.2, were observed in pachytene spermatocytes.Thus, it is apparent that TH2A synthesis in pachytene spermatocytes is not solely for replacement of the somatic H2A forms by a germ cell-specific variant in chromatin.The testis-enriched histone, X2 (Branson, R. E.,