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First published online 21 September 2004
doi: 10.1242/jcs.01368

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

¹ MGC-Department of Cell Biology and Genetics, Center for Biomedical Genetics, Erasmus MC, University Medical Center, PO Box 1738, 3000 DR Rotterdam, The Netherlands
² Department of Reproduction and Development, Erasmus MC, University Medical Center, PO Box 1738, 3000 DR Rotterdam, The Netherlands
³ Department of Obstetrics and Gynecology, University Medical Center St Radboud, PO Box 9101, 6500 HB Nijmegen, The Netherlands

^* Author for correspondence (e-mail: w.baarends{at}erasmusmc.nl)

Accepted 16 June 2004

In replicative damage bypass (RDB) in yeast, the ubiquitinconjugating enzyme RAD6 interacts with the ubiquitin ligase RAD18. In the mouse, these enzymes are represented by two homologs of RAD6, HR6a and HR6b, and one homolog of RAD18, Rad18^Sc. Expression of these genes and the encoded proteins is ubiquitous, but there is relatively high expression in the testis. We have studied the subcellular localization by immunostaining Rad18^Sc and other RDB proteins in mouse primary spermatocytes passing through meiotic prophase in spermatogenesis. The highest Rad18^Sc protein level is found at pachytene and diplotene, and the protein localizes mainly to the XY body, a subnuclear region that contains the transcriptionally inactivated X and Y chromosomes. In spermatocytes that carry translocations for chromosomes 1 and 13, Rad18^Sc protein concentrates on translocation bivalents that are not fully synapsed. The partly synapsed bivalents are often localized in the vicinity of the XY body, and show a very low level of RNA polymerase II, indicating that the chromatin is in a silent configuration similar to transcriptional silencing of the XY body. Thus, Rad18^Sc localizes to unsynapsed and silenced chromosome segments during the male meiotic prophase. All known functions of RAD18 in yeast are related to RDB. However, in contrast to Rad18^Sc, expression of UBC13 and pol, known to be involved in subsequent steps of RDB, appears to be diminished in the XY body and regions containing the unpaired translocation bivalents. Taken together, these observations suggest that the observed subnuclear localization of Rad18^Sc may involve a function outside the context of RDB. This function is probably related to a mechanism that signals the presence of unsynapsed chromosomal regions and subsequently leads to transcriptional silencing of these regions during male meiotic prophase.

Key words: Replicative damage bypass, Ubiquitination, Rad18^Sc, HR6a, HR6b, Meiosis, XY body

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