UBIQUITINATION OF HISTONE H3 IN ELONGATING SPERMATIDS OF RAT TESTES

Because of the potential role of histone ubiquitination in altering ch romatin structure, we characterized the levels of ubiquitination of sp ecific histones in meiotic and postmeiotic germ cells in rat testes by two-dimensional gel electrophoresis, The levels of the major ubiquiti nated histone forms, mono-and poly-ubiquitinated H2A, were highest in the pachytene spermatocyte stage, declined thereafter through the roun d spermatid stage, and reached their lowest levels in elongating sperm atids. Three additional ubiquitinated histone species, besides H2A, we re detected using anti-ubiquitin antibodies specifically in the fracti on enriched in elongating spermatids. Based on their electrophoretic m obilities, they corresponded to uH3, uTH3, and uH2B. Polyubiquitinated forms of these proteins were also observed. The identity of these pro teins was confirmed by immunoblotting with anti-H3 antisera and by dif ferential extraction of the proteins from the nucleus with increasing salt concentrations. This is the first report of ubiquitination of H3 in vivo. We speculate that its ubiquitination could loosen the nucleos ome structure in preparation for histone removal, be a consequence of nucleosome relaxation or disruption caused by other means, or target H 3 for degradation.