Substitutions in the protease (3Cpro) gene of poliovirus can suppress a mutation in the 5' noncoding region.

The poliovirus mutant 5NC-11 has a 4-base insertion at position 70 within the 5' untranslated region and is deficient in RNA synthesis. Revertants from 5NC-11 were isolated, showing a partial recovery of wild-type levels of RNA synthesis. The 5' noncoding region of those revertants contained the mutation intact; mix-and-match experiments with the cDNA from these revertants revealed that a restricted region within the 3C gene was the site of the suppressing mutations in the revertants. The suppressors were point mutations, confirmed by introducing them into the 3C gene by site-directed mutagenesis. Although complementation studies indicated that the suppressors were cis active, we believe that protein changes rather than RNA sequence alterations are responsible for the suppression because RNA changes that did not alter protein sequence had no effect, whereas various protein alterations were suppressive. The results therefore imply that protein 3C interacts with the 5' end of the RNA and may play a role in RNA replication.