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 rea rranged region of these pseudorevertants suggests that short direct re peats are strongly preferred as parting and anchoring sites during the jumps of the nascent strand 3' end. When the parting and anchoring si tes are separated by a long RNA segment, they can be brought in close proximity by an appropriate folding of the template strand. On the bas is of evidence derived from the analysis of the pseudorevertant genome s, it is proposed that a class of RNA rearrangements (some recombinati ons, deletions, insertions) proceed through the following steps: (i) p ausing of the nascent strand caused by misincorporations (or other rea sons); (ii) dissociation of the RNA polymerase together with the 3' en d of the nascent strand (a kind of proof-reading); and (iii) re-anneal ing of the nascent and template strands (precise or imprecise, but wit h the 3' base paired) and resumption of the synthesis.