The RNA-dependent RNA polymerase (RdRp) of potato virus X (PVX) contains a glycine-lysine-serine (GKS) motif. This motif is present in the replication enzyme of many RNA viruses and is thought to be required for nucleoside triphosphate-binding. Three single amino acid changes, glycine to alanine (AKS), lysine to asparagine (GNS) and lysine to glutamate (GES) within the GKS motif of the PVX RdRp were tested for their effect on PVX accumulation. The GNS and GES mutations rendered the virus unable to accumulate in either tobacco plants or protoplasts, whereas substitution of glycine with alanine had only a minor effect on accumulation of PVX. The glycine to alanine mutation reverted to wild-type after passage on Nicotiana clevelandii plants. These findings suggest that the GKS motif is required for PVX replication and that strong selection pressures are active to maintain necessary sequences of the viral RdRp.