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Helper Component Mutations in Nonconserved Residues Associated with Aphid Transmission Efficiency of a Pepper Isolate of Potato Virus Y

The aphid transmission properties of a pepper isolate of potato virus Y belonging to the pathotype 1--2 (PVY 1--2) have been characterized. PVY 1--2 was not transmitted in plant-to-plant experiments, although purified virus particles were efficiently transmitted when supplemented with heterologous helper component (HC) of the transmissible isolate PVY 0 AT through membrane acquisition assays, indicating that its coat protein was functional in transmission. Additionally, virions of PVY 1--2 were able to bind to different HCs in in vitro binding assays. Analysis of the sequence of the PVY 1--2 HC gene and comparison with that of PVY 0 AT revealed 19 nucleotide differences, but only 2 resulted in amino acid changes, one of which induced a change of charge. Neither of these two amino acid changes occurred within the cysteine-rich domain, nor did they coincide with conserved motifs of the HC protein known to be involved in aphid transmission and which are present in all known potyvi-ruses. However, both changes are located in positions highly conserved among PVY strains. The possible role of both mutations on the activity of the PVY 1--2 HC in aphid transmission is discussed.