During meiotic prophase in male mammals, the X and Y chromosomes are incorporated in the XY body. This heterochromatic body is transcriptionally silenced and marked by increased ubiquitination of histone H2A. This led us to investigate the relationship between histone H2A ubiquitination and chromatin silencing in more detail. First, we found that ubiquitinated H2A also marks the silenced X chromosome of the Barr body in female somatic cells. Next, we studied a possible relationship between H2A ubiquitination, chromatin silencing, and unpaired chromatin in meiotic prophase. The mouse models used carry an unpaired autosomal region in male meiosis or unpaired X and Y chromosomes in female meiosis. We show that ubiquitinated histone H2A is associated with transcriptional silencing of large chromatin regions. This silencing in mammalian meiotic prophase cells concerns unpaired chromatin regions and resembles a phenomenon described for the fungus Neurospora crassa and named meiotic silencing by unpaired DNA.Chromatin remodeling is at the basis of control of cellspecific gene expression, cell determination, and differentiation. The nucleosome units of chromatin consist of two each of the histones H2A, H2B, H3, and H4. The N-terminal ends of these core histones extend from the nucleosome and can undergo posttranslational covalent modifications, such as methylation, acetylation, phosphorylation, and ADP-ribosylation of specific amino acid residues. Together, these modifications constitute the so-called histone code (45). Interaction of other nuclear proteins with chromatin is dependent on the histone code at specific chromatin regions and determines local chromatin structure, which can be open or closed. A remarkable component of the histone code is ubiquitination of C-terminal lysine residues of histones H2A and H2B. Ubiquitin, a protein of 7 kDa, can be attached to lysine residues of a specific protein substrate through the action of a multienzyme complex containing ubiquitin-activating (E1), ubiquitin-conjugating (E2), and ubiquitin ligase (E3) enzymes. Polyubiquitination can target proteins for degradation by the proteasome (34, 35). Monoubiquitination of histones, however, is a stable modification that does not decrease the half-life of the target histone (56).In the yeast Saccharomyces cerevisiae, histone H2A ubiquitination is not required for cell growth or sporulation (47), but histone H2B ubiquitination is an essential mechanism involved in sporulation (37). Most importantly, it has been shown that ubiquitination of H2B by the ubiquitin-conjugating enzyme RAD6, interacting with the ubiquitin ligase BRE1, is a prerequisite for dimethylation of histone H3 at lysine residues 4 and 79 (5,12,37,46). This mechanism is thought to be associated with potentiation of gene activation. It is not known whether this "trans-histone" mechanism is conserved between yeast and mammals. RAD6 shows marked evolutionary conservation. The two mammalian homologs of yeast RAD6, Hr6a/Ube2a and Hr6b/Ube2b, both show approximately 70% amino...

In mammals, there is a complex and intriguing relationship between DNA repair and gametogenesis. DNA repair mechanisms are involved not only in the repair of different types of DNA damage in developing germline cells, but also take part in the meiotic recombination process. Furthermore, the DNA repair mechanisms should tolerate mutations occurring during gametogenesis, to a limited extent. In the present review, several gametogenic aspects of DNA mismatch repair, homologous recombination repair and postreplication repair are discussed. In addition, the role of DNA damage-induced cell cycle checkpoint control is considered briefly. It appears that many genes encoding proteins that take part in DNA repair mechanisms show enhanced or specialized expression during mammalian gametogenesis, and several gene knockout mouse models show male or female infertility. On the basis of such knowledge and models, future experiments may provide more information about the precise relationship between DNA repair, chromatin dynamics, and genomic stability versus instability during gametogenesis.

The ubiquitin system is involved in numerous cellular processes, regulating the amounts and/or activities of specific proteins through posttranslational coupling with ubiquitin or ubiquitin-like proteins. In spermatogenesis, there appears to be a special requirement for certain components of the ubiquitin system, as exemplified in human and mouse by mutation of USP9Y and HR6B, respectively. Both genes encode proteins which take part in the ubiquitin system and are ubiquitously expressed, but their mutation generates no apparent phenotype other than male infertility. Different phases of mammalian spermatogenesis probably require different specialized activities of the ubiquitin system. It is anticipated that ubiquitination activities similar to those required during mitotic cell cycle regulation will play some role in control of the meiotic divisions. In spermatocytes, there is an intricate link among DNA repair, the ubiquitin system, and regulation of meiotic chromatin structure, as indicated by the co-localization of proteins involved in these processes on meiotic recombination complexes. HR6B and its nearly identical homolog HR6A are multiple function proteins, with ubiquitin-conjugating activity and essential roles in post-replication DNA repair. HR6B, possibly together with the ubiquitin-ligating enzyme mRAD1 8Sc, is most likely involved in chromatin re-organization during the meiotic and post-meiotic phases of spermatogenesis. Biochemical data indicate that, in particular during spermiogenesis, the general activity of the ubiquitin system is high, which most likely is related to the high requirement for massive breakdown of cytoplasmatic and nuclear proteins during this last phase of spermatogenesis.

SummaryUbiquitin ligase Rad18 Sc localizes to the XY body and to other chromosomal regions that are unpaired and transcriptionally silenced during male meiotic prophase

No abstract