Elongation activity of poliovirus RNA polymerase derived from Sabin type 1 sequence is not temperature sensitive

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 (3Dpol) coding region. These mutations also contribute to a temperature sensitive pheno-type of virus replication. We have cloned and expressed the Sabin 1 virus 3Dpol) 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 degrees C. All three showed the same kinetics of heat inactivation after pre-incubation at elevated temperatures. Thus the contribution of Sabin 3Dpol 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.