RNA polymerase activity catalyzed by a potyvirus-encoded RNA-dependent RNA polymerase

We have expressed the putative RNA-dependent RNA polymerase encoded by the potyvirus tobacco vein mottling virus (TVMV) in Escherichia coli as a glutathione S-transferase fusion protein. As prepared, the fusion protein possessed the poly(U) polymerase activity that is a hallmark of other picornavirus-encoded polymerases. In addition, this protein was able to utilize full-length TVMV RNA as a template for RNA synthesis. A fusion protein containing a mutation in the highly conserved GDD motif of the polymerase (GDD-->ADD) possessed 7% of the activity of the wild type. Our results confirm that the presumed polymerase encoded by TVMV is in fact an RNA-dependent RNA polymerase and that the GDD motif so widely seen in viral polymerases has an important function in the TVMV protein.