Genetic recombination with poliovirus type 1: Studies of crosses between a normal horse serum-resistant mutant and several guanidine-resistant mutants of the same strain

Abstract

Mixed infection of HeLa cells with two mutants of type 1 poliovirus yielded progeny which were resistant to both inhibitors. One mutant ( ho) was resistant to the plaque-forming inhibitor in certain horse sera and the other was resistant to the virus inhibitory effects of guanidine. Mutants of poliovirus were isolated which had different degrees of resistance to the effects of guanidine. Some of these mutants were stable during repeated cultivation in HeLa cells in the absence of the drug. Mutants were also isolated which were dependent on guanidine for growth. Mixed infection with an ho mutant (resistant to the plaque inhibitory effects of horse serum) and a g 100 mutant (resistant to the inhibitory effects of 100 μg of guanidine per milliliter in agar overlay) under conditions of high and approximately equal multiplicity of infection yielded progeny containing 0.42% of the recombinant mutant ho g 100 . All double mutants tested possessed the full resistance to guanidine. Mixed infection with mutants ho and g 20 or ho and g 27 yielded approximately 0.4% of the recombinants ho g 20 or ho g 27 . While the spontaneous rate of g mutants to ho g was fairly high, the proportion of double mutants found was 15–20 times that expected as a result of mixed infection and, by crossing g 100 with a strain ho Δ r, the occurrence of heat-resistant clones in the recombinant progeny ruled out the possibility that the spontaneous mutation was the sole source of the double mutants. The frequency of ho g recombinants increased with time after mixed infection. The first new virus contained about 0.2% recombinants, and this proportion increased to about 0.4% at the time of complete virus maturation. This result suggests that RNA produced in the cell as the result of infection may act as a template for further RNA production and may recombine so that the RNA produced may be recombinant.