Determinants of attenuation in the Sabin type 1 strain of poliovirus are located in the 5′ noncoding region, the capsid coding region and the viral RNA-dependent RNA polymerase (3D) coding region. These mutations also contribute to a temperature sensitive phenotype of virus replication. We have cloned and expressed the Sabin 1 virus 3D protein which contains three amino acid differences from the wild-type (Mahoney) sequence, as well as a wild-type polymerase containing only a single Sabin amino acid substitution at nt 6203. These enzymes have been examined and compared for temperature sensitive polymerase activity. Wild-type and mutated polymerases demonstrated identical specific activities at 30, 35 and 39 °C. All three showed the same kinetics of heat inactivation after pre-incubation at elevated temperatures. Thus the contribution of Sabin 3D sequences to the inability of the virus to grow at elevated temperatures must lie in a function or activity of the enzyme other than RNA polymerization. A likely reaction is the initiation step of RNA chain synthesis.


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