Figure 4
From: Conserved structure and inferred evolutionary history of long terminal repeats (LTRs)
A model for LTR retrotransposon evolution. The figure is an attempt to reconstruct a parsimonious sequence of events leading to LTR retrotransposons. It is based on the RT trees shown in Figure 3 and Additional file 2: Figure S4. Five stages of LTR genesis are postulated: 1. Addition of LTR-like terminal repeats which 2. can hairpin prime, 3. A promoter structure next to one of these repeats, in the vicinity of a polyadenylation signal/site. 4. A DDE integrase, acquired in several independent events. 5. A PBS which replaced the hairpin primer. This led to full LTR function. The addition of capsid, protease and envelope protein genes are also marked. PAS, Polyadenylation signal and site; PBS, Primer binding site; RH, RNAse H; RT, Reverse transcriptase; RV, Retroviruses (Classes I, II and III); TSS, Transcriptional start site.