GENOMICS Vol. 79, Number 3, March 2002 Copyright © 2002 Elsevier Science (USA). All rights reserved. 0888-7543/02 $35.00 387 Article doi:10.1006/geno.2001.6715, available online at http://www.idealibrary.com on IDEAL INTRODUCTION The phospholipid hydroperoxide glutathione peroxidase (PHGPx) has been characterized as an unusual isoform of selenium-dependent glutathione peroxidases [1] that is capa- ble of reducing ester lipid hydroperoxides, such as hydroper- oxy phospholipids or hydroperoxy cholesterol esters [2–4], to their less toxic hydroxy derivatives. Like other glutathione peroxidases [5,6], PHGPx has been implicated in defense mechanisms protecting cells from oxidative stress, and sev- eral lines of experimental evidence support this hypothesis [7–10]. On the other hand, the tissue distribution [11] and the regulation of PHGPx expression [12,13] may indicate a more specific role of the enzyme in cell development and differen- tiation, particularly in spermatogenesis [14–16]. In addition, the enzyme may be of regulatory importance for the activity of enzymes involved in the arachidonic acid cascade [17–20]. The cDNAs for various mammalian PHGPx isoforms have been cloned [21–24], and the complete sequence of the porcine [25] and human [26] PHGPx genes are known. PHGPx con- tains a catalytically active selenocysteine, which is encoded by a TGA stop codon. In mammalian cells the stop of translation at this TGA is prevented by the selenocysteine insertion sequence (SECIS) localized in the 3-UTR of the PHGPx mRNA [21–24,27,28]. Deletion or truncation of this sequence Discovery of a Functional Retrotransposon of the Murine Phospholipid Hydroperoxide Glutathione Peroxidase: Chromosomal Localization and Tissue-Specific Expression Pattern Carolin Boschan, Astrid Borchert, Christoph Ufer, Bernd-Joachim Thiele, and Hartmut Kuhn* Institute of Biochemistry, University Clinics Charité, Humboldt University, Monbijoustr.2, D-10117 Berlin, Germany *To whom correspondence and reprint requests should be addressed. Fax: (+49) 30450 528905. E-mail: hartmut.kuehn@charite.de. Phospholipid hydroperoxide glutathione peroxidase (PHGPx), a selenoprotein capable of reducing toxic hydroperoxy ester lipids, has been implicated in antioxidative defense and spermatogenesis. Screening a murine genomic library, we isolated two recombinants (pseudo- genes 1 and 2) containing retrotransposons for this enzyme. On comparison with the paral- ogous cDNA, pseudogene 1 contained only two silent nucleotide exchanges, and the 3-untranslated region (3-UTR) carrying the functionally important selenocysteine insertion sequence was free of mutations. This retrotransposon was found in various mouse strains and could be mapped to the region B2–B3 of chromosome 10. In vitro studies indicated signifi- cant promoter activity in the 5-flanking region of pseudogene 1, and we observed a tissue- specific expression of this retrotransposon. In the submandibular gland. Most PHGPx tran- scripts originated from pseudogene 1. In contrast, pseudogene 2, containing numerous mutations in all parts of the retrotransposon, was not expressed in any tissue. It was mapped to region E3–E4 of chromosome 17, and we did not detect any promoter activity in its 5-flanking region. These data indicate the existence of two retrotransposed PHGPx pseudo- genes, one of which encodes a functional enzyme. This retrotransposon belongs to the rare group of pseudogenes that are tissue-specifically expressed under the control of captured reg- ulatory elements, and it constitutes an example of evolutionarily acquired redundancy in gene expression. The results are important for the design of future knockout strategies aimed at investigating the biological role of this enzyme. Key Words: oxidative stress, peroxides, murine genome, gene expression, spermatogenesis, lipid peroxidation, selenium