Histone ubiquitination and chromatin remodeling in mouse spermatogenesis

Male infertility in HR6B knockout mice is associated with impairment of spe rmatogenesis. The HR6B gene is a mammalian, autosomal homolog of the Saccha romyces cerevisiae gene Rad6 encoding a ubiquitin-conjugating enzyme. In ad dition, X-chromosomal HR6A has been identified, in human and mouse. RAD6 in yeast is required for a variety of cellular functions, including sporulati on, DNA repair, and mutagenesis. Since RAD6 and its mammalian homologs can ubiquitinate histones in vitro, we have investigated the pattern of histone ubiquitination in mouse testis. By immunoblot and immunohistochemical anal ysis of wild-type mouse testis, a high amount of ubiquitinated H2A (uH2A) w as detected in pachytene spermatocytes. This signal became undetectable in round spermatids, but then increased again during a relatively short develo pmental period, in elongating spermatids. No other ubiquitinated histones w ere observed. In the HR6B knockout mice, we failed to detect an overt defec t in the overall pattern of histone ubiquitination. For somatic cell types, it has been shown that histone ubiquitination is associated with destabili zation of nucleosomes, in relation to active gene; transcription. Unexpecte dly, the most intense uH2A signal in pachytene spermatocytes was detected i n the sex body, an inactive nuclear structure that contains the heterochrom atic X and Y chromosomes. The postmeiotic uH2A immunoexpression in elongati ng spermatids indicates that nucleosome destabilization induced by histone ubiquitination may play a facilitating role during histone-to-protamine rep lacement. (C) 1999 Academic Press.