MUTATIONAL ANALYSIS OF THE HELPER COMPONENT-PROTEINASE GENE OF A POTYVIRUS - EFFECTS OF AMINO-ACID SUBSTITUTIONS, DELETIONS, AND GENE REPLACEMENT ON VIRULENCE AND APHID TRANSMISSIBILITY

We have previously provided evidence that amino acid substitutions wit hin the N-terminal portion of the helper component-proteinase (HC-Pro) from tobacco vein mottling virus (TVMV), in particular at Lys-307, no t only affect the aphid transmission activity of HC-Pro but also have a significant effect on TVMV virulence. In the present study amino aci ds which differ in their charge properties were substituted at positio n 307. A highly basic residue was required to retain helper component activity and virulence. Deletion and insertion mutagenesis in the 5' t erminus of the HC-Pro gene suggested that this RNA domain may be an es sential element for TVMV infectivity. Replacement of the TVMV HC-Pro g ene with that from another potyvirus, zucchini yellow mosaic virus, ma intained infectivity and aphid transmissibility of the chimeric virus, although symptoms were attenuated. Our results suggest that, in addit ion to its importance in aphid transmission, the HC-Pro gene may be of general importance in regulating virulence of potyviruses, possibly b y interaction of these sequences with the host.