Fig. 3. The MLV packaging signal contains a riboswitch.

All retroviral vRNAs are packaged into virus particles in dimeric form. In MLV, the contacts between the two monomers in the dimer (ii) include the extended base-paired stretches “PAL1” and “PAL2” (green and red boxes), and “kissing-loop” interactions between SL1 in one monomer and SL2 in the other (orange boxes). Each monomer contains two UCUG-UPu-UCUG motifs (blue boxes). Dimerization involves a “switch”, in that the UCUG-UPu-UCUG motifs are partially or fully base-paired in the monomer (i), but unpaired in the dimer. They are high-affinity binding sites for MLV Gag protein and are bound by NC protein in mature MLV particles; replacement of the four G residues with A residues prevents selective packaging of vRNA [50-52].