Meiotic synthesis of testis histones in the rat.

The distribution and synthesis of the testis-specific variants of histones H1 and H2B, TH1 and TH2B, respectively, and of the somatic histones were studied in rat testis cells. Rat testis cells were labeled in vivo with intratesticular injections of [3H]lysine. The cells and nuclei were then separated into different developmental classes by velocity sedimentation and the histones were analyzed. TH1 and TH2B, as well as the somatic histones, were present in spermatocytes and round spermatids, but none of them were detectable in elongated spermatids. The synthesis and nuclear accumulation of TH1 and TH2B took place throughout pachytene, as well as in earlier stages, but not in the round spermatids. In addition, there was synthesis during pachytene of a histone that migrates electrophoretically with H2A. However, somatic histone synthesis, with the possible exception of H2A and H2B, was not detectable at the pachytene stage. In vivo treatment of rats with hydroxyurea reduced DNA synthesis in the testis to 1% of control values and significantly reduced the synthesis of H3, H2B, H2A, and H4, with the greatest effect being on H3 and H4. However, the hydroxyurea treatment did not significantly decrease the synthesis of TH1, H1, or TH2B. These results prove that the synthesis of several histones during the meiotic prophase is not dependent upon concurrent S-phase DNA synthesis.