Revertants of temperature-sensitive mutants of reovirus: Evidence for frequent extragenic suppression

Abstract

Twenty-eight independently isolated, spontaneous revertants isolated from temperature-sensitive (ts) mutants of reovirus type 3 representing all the known mutant groups, were examined to determine whether they were intragenic revertants or contained extragenic suppressor mutations. Analysis of the progeny of backcrosses of the revertants to wild type, showed that 25 of the 28 revertants contained is lesions. This result indicated that 25 of the 28 revertants were suppressed pseudorevertants with the suppressor mutation in a gene that could be separated from the parental is lesion by recombination. The nature of the is lesion(s) was examined for a number of the is clones derived from back-crosses. In every case, except one, the parental is lesion was found. In five of the ten suppressed pseudorevertants examined, nonparental is lesions could also be rescued. Two of the nonparental is lesions were in the previously defined recombinant groups. Five of the nonparental is lesions represented a new recombination group or groups since they recombined with the prototype mutants of all of the defined recombination groups. Recombination analysis indicated that the five new mutants fall into two recombination groups for which we propose the designations H and I. The nonparental is lesions rescued from suppressed pseudorevertants may represent suppressor mutations with is phenotype. However, the majority of the suppressor mutations identified had no temperature phenotype and were identified only by their effect on the phenotype of the original is lesion. The fact that a large proportion of revertants were suppressed by extragenic suppressor mutations suggests that mutation events leading to extragenic suppression occur at a much higher frequency than do intragenic events leading to revertant phenotype. These results indicate a general mechanism by which RNA viruses can bypass the effects of deleterious mutations in the absence of intramolecular recombination.