DIFFERENT HELPER COMPONENT MUTATIONS ASSOCIATED WITH LACK OF APHID TRANSMISSIBILITY IN 2 ISOLATES OF POTATO-VIRUS-Y

Two potato virus Y (PVY) isolates, PVY-0 NAT and PVY-1, were character ized as aphid-nontransmissible in plant-to-plant transmission tests. M embrane and sequential transmission tests determined that their lack o f transmissibility was due to a defect in their helper component (HC) proteins. A comparative analysis of the HC genes of the nontransmissib le isolates PVY-0 NAT and PVY-1 with a transmissible isolate, PVY-0, s howed that a single amino acid substitution differentiated the HC of i solate PVY-0 NAT from that of isolate PVY-0. The substitution was loca ted within a cysteine-rich region at the amino-terminal region of the protein, in the same position as mutations found in other HC-deficient potyviruses. However, the sort of exchange differed (Lys to Glu for p otato virus C and for isolates R5A and Connecticut of zucchini yellow mosaic virus; Lys to Asn for isolate PVY-0 NAT). On the other hand, th e HC of isolate PVY-1 had two amino acid substitutions; one of them (G ly to Asp) was also within the cysteine-rich region, and the other (Se r to Gly) was in the carboxy-terminal region of the protein. These aph id-nontransmissible isolates presented different HC-related properties . While the HC protein was detected in partially purified preparations obtained from PVY-0 NAT-infected plants, it could not be detected in equivalent preparations obtained from PVY-1-infected plants, even thou gh the presence of an HC in these plants was demonstrated by serologic al means. In addition, the HC of this viral isolate showed an electrop horetic behavior distinct from those shown by the HCs of other PVY iso lates. The relationships between the nucleotide sequence data and thes e HC-related properties are discussed.