Isolation and biological characterization of 3(2H)-isoflavene-resistant and -dependent poliovirus type 2 Sabin mutants

Poliovirus type 2 Sabin mutants were selected for drug resistance and depen dence by plating on HeLa cell monolayers in the presence of 3(2H)-isoflaven e, a compound related to dichloroflavan, which prevents the shut-off of hos t translation and poliovirus RNA and protein synthesis. The drug-resistant mutants grew equally well in the presence and in the absence of the drug, w hile the drug-dependent mutants only grew in the presence of the compound. One dependent and one resistant mutant were characterized biologically in m ore detail. The resistant mutant did not exhibit thermolability. The mild t hermolability exhibited by the dependent mutant was not affected by the add ition of 3(2H)-isoflavene, indicating that the substance does not bind the poliovirus type 2 Sabin capsid, The translation of viral proteins and the s hut-off of host protein translation during cell infection were not inhibite d in either mutant. In the absence of the drug, the cleavage of the precurs or VPO, a step in virus protein processing, was affected in the dependent m utant. The dependence of the mutant on the drug was due to the inability of 75S empty particles to reach maturation: our results strongly suggest that this phenomenon is strictly dependent on the reduction of RNA synthesis, c onfirming the existence of a dynamic equilibrium between RNA production and genome encapsidation during the poliovirus replication cycle.