MUTATION OF THE GKS MOTIF OF THE RNA-DEPENDENT RNA-POLYMERASE FROM POTATO-VIRUS-X DISABLES OR ELIMINATES VIRUS-REPLICATION

The RNA-dependent RNA polymerase (RdRp) of potato virus X (PVX) contai ns a glycine-lysine-serine (GKS) motif. This motif is present in the r eplication enzyme of many RNA viruses and is thought to be required fo r nucleoside triphosphate-binding. Three single amino acid changes, gl ycine to alanine (AKS), lysine to asparagine (GNS) and lysine to gluta mate (GES) within the GKS motif of the PVX RdRp were tested for their effect on PVX accumulation. The GNS and GES mutations rendered the vir us unable to accumulate in either tobacco plants or protoplasts, where as substitution of glycine with alanine had only a minor effect on acc umulation of PVX. The glycine to alanine mutation reverted to wild-typ e after passage on Nicotiana clevelandii plants. These findings sugges t that the GKS motif is required for PVX replication and that strong s election pressures are active to maintain necessary sequences of the v iral RdRp.