Characterization of the low-copy HERV-Fc family: evidence for recent integrations in primates of elements with coding envelope genes.

In a previous search based on the envelope gene, we had identified two related proviral elements that could not be included in identified ERV families. An in silico database screening associated with an in vivo polymerase chain reaction search using primers in the reverse transcriptase domain, now allowed identification of a series of related elements, found at a limited number in simians. A phylogenetic analysis led to their inclusion in a new family of endogenous retroviruses with limited expansion, which we named ERV-Fc, and which is part of the enlarged ERV-F/H family. The human genome comprises only six HERV-Fc, among which two possess full-length coding envelope genes. A complete provirus was identified in the baboon, also disclosing a fully open envelope gene. Cloning of the sites orthologous to the envelope-coding human proviruses demonstrated presence of the integrated proviruses in chimpanzee and gorilla, but not in orangutan. For the baboon element, the orthologous locus was found empty even in the phylogenetically most closely related macaque, again suggesting, together with the complete identity of its LTRs, "recent" integration. The data presented are compatible with an evolutionary scheme in which the ERV-Fc proviruses would be the endogenous traces of an active retroviral element, possibly acting as an infectious retrovirus with low endogeneization potency, with evidence for integrations at two distinct periods of primate evolution.