Genetic complementation among poliovirus mutants derived from an infectious cDNA clone.

We constructed several well-defined mutations in the nonstructural portion of the poliovirus type I (Mahoney strain) genome by making small insertions in an infectious cDNA clone. The derived viral strains carrying the mutations exhibited a variety of distinct plaque phenotypes. Thus, we were able to examine genetic complementation between different pairs of mutants by comparing the yields of progeny virus in mixed and single infections. Two mutants bearing lesions in the 2A and 3A regions of the genome, which are defective in the inhibition of host cell translation and the synthesis of viral RNA, respectively, could be rescued efficiently by genetic complementation; three replication-deficient mutants containing insertions in the 2B, 3D (replicase), and 3'-untranslated regions could not. Both the 2A and 3A mutants could be rescued by each other and by all of the other mutants tested. Because yield enhancement was apparent well before the completion of a single infectious cycle, it is likely that complementation of both mutants involved early diffusion of functional products. These data provide the first unambiguous evidence that the nonstructural portion of the poliovirus genome contains multiple complementation groups. The data also suggest that certain nonstructural functions act only in cis.