MUTATIONS IN THE HELPER COMPONENT PROTEASE GENE OF ZUCCHINI YELLOW MOSAIC-VIRUS AFFECT ITS ABILITY TO MEDIATE APHID TRANSMISSIBILITY

The nucleotide sequence of the helper component protease (HC-Pro) gene s of three zucchini yellow mosaic virus (ZYMV) strains has been compar ed with that of a helper-deficient strain of ZYMV-HC. The comparisons revealed three unique deduced amino acid differences. Two of these mut ations were located in regions which are conserved in other potyviruse s. The role of these mutations in aphid transmissibility was examined by exchanging DNA fragments of part of the deficient HC-Pro gene with the respective section within the gene of the infectious full-length c lone of the aphid-transmissible ZYMV. The first exchange included two of the three mutations, the first coding for a change from Asp to Gly (in a non-conserved region) and the second coding for a change from Ar g to Ile [within the Phe-Arg-Asp-Lys (FRNK) conserved box]. This excha nge resulted in a reduced transmission (20.6% for the mutated virus co mpared with 57.4% in the normal ZYMV when acquired from plants and 37. 2% compared with 83.1%, respectively, when acquired from membranes). T he second exchange incorporated a single mutation [conferring a change from Thr to Ala within the Pro-Thr-Lys (PTK) conserved box]. This sin gle mutation resulted in almost total loss of HC activity in aphid tra nsmission both from plants and from membranes. The Lys residue in the conserved Lys-Ile-Thr-Cys (KITC) box, which is related to loss of HC a ctivity in potato virus Y, tobacco vein mottling virus and in the Mich igan strain of ZYMV, is unchanged in the helper-deficient ZYMV. It is therefore proposed that more than one site in HC-Pro may be functional ly related to aphid transmissibility. The possible reasons for the rol e of these mutations in helper activity in aphid transmission of ZYMV are discussed.