EXPRESSION OF THE UBIQUITIN-CONJUGATING DNA-REPAIR ENZYMES HHR6A AND HHR6B SUGGESTS A ROLE IN SPERMATOGENESIS AND CHROMATIN MODIFICATION

RAD6, a member of the expanding family of ubiquitin-conjugating (E2) e nzymes, functions in the so-called ''N-rule'' protein breakdown pathwa y of Saccharomyces cerevisiae. In vitro, the protein can attach one or multiple ubiquitin (Ub) moieties to histones H2A and B and trigger th eir E3-dependent degradation. Rad6 mutants display a remarkably pleiot ropic phenotype, implicating the protein in DNA damage-induced mutagen esis, postreplication repair, repression of retrotransposition, and sp orulation. RAD6 transcription is strongly induced upon UV exposure and in meiosis, suggesting that it is part of a damage-induced response p athway and that it is involved in meiotic recombination. It is postula ted that the protein exerts its functions by modulating chromatin stru cture. Previously, we have cloned two human homologs of this gene (des ignated HHR6A and HHR6B) and demonstrated that they partially compleme nt the yeast defect. Here we present a detailed characterisation of th eir expression at the transcript and protein levels. Both HHR6 protein s, resolved by 2-dimensional immunoblot analysis, are expressed in all mammalian tissues and cell types examined, indicating that both genes are functional and constitutively expressed. Although the proteins ar e highly conserved, the UV induction present in yeast is not preserved , pointing to important differences in damage response between yeast a nd mammals. Absence of alterations in HHR6 transcripts or protein upon heat shock and during the cell cycle suggests that the proteins are n ot involved in stress response or cell cycle regulation. Elevated leve ls of HHR6 transcripts and proteins were found in testis. Enhanced HHR 6 expression did not coincide with meiotic recombination but with the replacement of histones by transition proteins. Immunohistochemistry d emonstrated that the HHR6 proteins are located in the nucleus, consist ent with a functional link with chromatin. Electron microscopy combine d with immunogold labeling revealed a preferential localisation of HHR 6 in euchromatin areas, suggesting that the protein is associated with transcriptionally active regions. Our findings support the idea that both HHR6 genes have overlapping, constitutive functions related to ch romatin conformation and that they have a specific role in spermatogen esis, involving Ub-mediated histone degradation. (C) 1996 Academic Pre ss, Inc.