A MODEL FOR REARRANGEMENTS IN RNA GENOMES

Engineered mutants of Theiler's murine encephalomyelitis virus (TMEV) and poliovirus having altered spacing between the oligopyrimidine and AUG moieties of a translational control element are known to generate pseudorevertants with deletions or insertions that tend to restore the wild-type structure of this element. The primary structure of the rearranged region of these pseudorevertants suggests that short direct repeats are strongly preferred as parting and anchoring sites during the jumps of the nascent strand 3' end. When the parting and anchoring sites are separated by a long RNA segment, they can be brought in close proximity by an appropriate folding of the template strand. On the basis of evidence derived from the analysis of the pseudorevertant genomes, it is proposed that a class of RNA rearrangements (some recombinations, deletions, insertions) proceed through the following steps: (i) pausing of the nascent strand caused by misincorporations (or other reasons); (ii) dissociation of the RNA polymerase together with the 3' end of the nascent strand (a kind of proof-reading); and (iii) re-annealing of the nascent and template strands (precise or imprecise, but with the 3' base paired) and resumption of the synthesis.