Poliovirus replicase stimulation by terminal uridylyl transferase.

In an in vitro poliovirus replication system, purified viral polymerase, plus sense virion RNA, and a host factor have been previously shown to be necessary for the transcription of minus strands. We have found that a partially purified eukaryotic initiation factor-2 (eIF-2) fraction from rabbit reticulocytes can replace HeLa host factor in the replicase reaction. This enzyme preparation contains eIF-2 and two other major proteins. In addition to eIF-2 activity, which does not appear to play a role in the replicase reaction, we find that the fraction contains terminal uridylyl transferase activity. The enzyme adds UMP moieties to the 3' end of primer RNA molecules. The number of UMP residues added depends on the primer. Although long tails of heterogeneous lengths (50 to 100 nucleotides) can be polymerized on the 3' end of oligo(U), a poly(A) primer accepts only four U's. The terminal uridylyl transferase activity requires only UTP, Mg2+, a sulfhydryl reagent, and an RNA primer for activity. It is partially associated with ribosomes. We provide preliminary evidence that it may be responsible for host factor-like activity. We present a model for minus strand synthesis by poliovirus replicase, based on the hypothesis that a terminal uridylyl transferase can participate in initiation.