Figure 3.

Model for processing in vivo of the oligomeric (+) replicative intermediates of the family Pospiviroidae that involves a kissing loop interaction between the palindromic tetraloops of two consecutive hairpin I motifs (A), with their stems forming subsequently a longer interstrand duplex (B). This double-stranded structure is the substrate for cleavage at specific positions in both strands (C). Following a second conformational switch the resulting unit-length strands adopt the extended rod-like structure with loop E (in outlined fonts) and the adjacent bulged-U helix (D), which is the substrate for ligation (E). R and Y refer to purines and pyrimidines, respectively, the S-shaped line denotes the UV-induced cross-link, and white arrowheads mark the cleavage sites in the double-stranded structure and the ligation site in the extended conformation. Reprinted with permission from [26].