Fig. 2
From: The impact of transposable element activity on therapeutically relevant human stem cells
Retrotransposons in the human genome. Currently, only LINE-1 (L1), Alu and SVA elements are verifiably still mobilized in humans. A full length HERV-K provirus is ~ 9.5 kb long, codes for group-specific antigen (Gag), protease (Pro), polymerase (Pol) and envelope (Env) proteins and is flanked by ~ 1-kb long terminal repeats (LTRs) with the 5’LTR including the HERV-K promoter. A functional full length L1 element is ~ 6 kb in length, harbours three open reading frames (ORF0, ORF1 and ORF2) at which ORF1 and ORF2 are separated by a 63-bp noncoding spacer region. The 5′ untranslated region (5’UTR) harbours both sense and antisense promoter. Alu elements comprise ~ 280-300 bp, are composed of two 7SL-RNA derived monomers separated by an A-rich connector (A5TACA6), contain an internal A and B box promoter, and end in a poly A tail (An). SVA elements are ~ 0.7–4 kb long, consist of a 5′ hexamer repeat that can be variable in length ((CCCTCT)n), two Alu fragments in antisense orientation, a GC-rich variable number of tandem repeats (VNTR) region, a SINE-R sequence derived from an HERV-K10 element and a poly A tail following a polyadenylation signal. The length of an intact SVA can vary depending on the number of repeats present in the hexamer and VNTR domains. L1, Alu and SVA insertions are characterized by the hallmarks of L1-mediated retrotransposition such as flanking variable target site duplications (TSDs), polyA tails at their 3′ ends (An) and insertion at the consensus target sequence 5′-TTTT/AA-3′. 3’UTR, 3′ untranslated region