Poliovirus RNA polymerase mutation 3D-M394T results in a temperature-sensitive defect in RNA synthesis

Mutant ts10 is an RNA-negative temperature-sensitive mutant of Mahoney type 1 poliovirus. Mutant ts10 3D pol was purified from infected cells and was shown to be rapidly heat-inactivated at 45 degrees when compared to wild-type polymerase. Sequencing of mutant ts10 genomic RNA revealed a U to C transition at nt 7167 resulting in an amino acid change of methionine 394 of 3D pol to threonine. The 3D-M394T mutation was engineered into a wild-type infectious clone of poliovirus type 1. The resultant mutant virus, 3D-105, had a temperature-sensitive phenotype in plaque assays. The translation and replication of wild-type, ts10, and 3D-105 virion RNAs were all characterized in HeLa S10 translation-RNA replication reactions in vitro. The optimum temperatures for the replication of the wild-type and mutant viral RNAs in the HeLa S10 translation-replication reactions were 37 and 34 degrees, respectively. To characterize the temperature-sensitive defect in the replication of the mutant RNA, we used preinitiation RNA replication complexes which were formed in HeLa S10 in vitro reactions containing guanidine HCl. Negative-strand RNA synthesis in 3D-M394T mutant preinitiation replication complexes was normal at 34 degrees but was rapidly and irreversibly inhibited at 39.5 degrees. To differentiate between the initiation and elongation steps in RNA replication, we compared the elongation rates in mutant and wild-type replication complexes at 39.5 degrees. The results showed that the elongation rates for nascent negative strands in both the mutant and wild-type replication complexes were identical. Therefore, the results indicate that the heat-sensitive step in negative-strand synthesis exhibited by the 3D-M394T replication complexes is in the initiation of RNA synthesis and not in the elongation of nascent chains.