Human and mouse XAP-5 and XAP-5-like (X5L) genes: Identification of an ancient functional retroposon differentially expressed in testis

Although most retroposons that arose by reverse transcription of cellular m RNAs and by reintegration into the genome are nonfunctional, several exampl es exist in which the retroposon acquired a novel function and became fixed in the genome as a functional gene. We identified another such case: the u biquitously expressed X-linked XAP-5 gene with unknown function gave rise t o its retroposed counterpart, XAP-5-like (X5L), which has an intronless ope n reading frame and is autosomal in human. Phylogenetic analysis of the hum an and mouse XAP-5 and X5L genes shows that the retroposition most likely t ook place before the radiation of eutherian mammals. The XAP-5 and X5L gene s are expressed in a wide range of tissues but are differentially expressed in testis. The ancient origin and broad expression of the X5L retroposon i ndicate that the XAP-5 and X5L genes may have assumed different functions i n somatic cells. In addition to this, because of its autosomal location and its high level and particular pattern of expression in spermatogenic cells , the X5L expression in testis may compensate for the X-linked XAP-5 gene, which may be silenced during spermatogenesis. (C) 1999 Academic Press.