Figure 3.

Exemplified HERV precursor genomic structure. This proviral structure is based on HERV-K(HML2) proviruses, which are considered the most recently introduced in humans (in the last five million years) and contain the most complete proviral copies, including full-length proviruses, with the most intact open reading frames (ORFs). The proviral LTRs can act as promoters of RNA transcription. Four major ORFs are depicted: gag encoding structural proteins; pro encoding protease; pol encoding RT, RNAse H, and IN domains; and env encoding for the retroviral envelope proteins. A smaller ORF, termed rec, is the functional counterpart of Rev and Rex encoded by more complex retroviruses, such as HIV-1 and HTLV-1, respectively. There appear to be several-hundred copies of proviruses belonging to this particular group in the human genome. However, in addition to very few copies of the near complete structure shown here, the majority harbor deletions ranging from small internal deletions to just solo LTRs, proviruses which have lost, by recombination between the two LTRs, all integral genes, leaving a single LTR sequence.