Dependence of the activity of the poliovirus replicase on the host cell protein.

Two poliovirus-specific RNA polymerase activities have been identified: a poly(U) polymerase that copies poly(A).oligo(U) and a replicase that copies natural heteropolymers with some preference for poliovirus RNA. Both activities purified together until a step of a salt gradient elution from poly(U)-agarose, when poly(U) polymerase but no replicase was recovered. Addition of a salt wash fraction from the ribisomes of uninfected cells to this poly(U) polymerase fraction reconstituted replicase activity, and a host factor was purified 50 fold from the ribosomal salt wash. None of the available initiation factors or elongation factors for protein synthesis were able to reconstitute replicase activity. Host factor activity could be supplied by adding oligo(U), suggesting that the factor acts at the initiation step of RNA replication. With the purified replicase-host factor combination, only poly(A)-containing RNAs were copied, and a preference for poliovirus RNA was shown.