The va gene is used in commercial Burley tobacco cultivars including cv TN 86 to confer resistance to tobacco vein mottling virus (TVMV) and, to some extent, other potyviruses. A naturally occurring strain of TVMV (TVMV-S), which overcomes this resistance, was isolated from TN 86 plants. To investigate the mechanism by which TVMV-S overcomes va gene resistance, a cDNA clone encompassing the complete genome of TVMV-S was produced. Using chimeric transcripts combining regions of TVMV-S and regions of the wild-type strain (TVMV-WT) to which TN 86 is resistant, it was demonstrated that a domain within the VPg protein is responsible for overcoming va resistance in TN 86. DNA sequence analysis revealed six amino acid differences between the two strains of TVMV within this domain. Inclusion of sequences for four of the TVMV-S VPg amino acids was sufficient to confer the resistance-overcoming phenotype to all corresponding transcripts. Coinoculation experiments suggested that the resistance of TN 86 to TVMV-WT was not due to the induction of a general host defense response. The results are compatible with the hypothesis that VPg must assume an appropriate configuration in order to interact with appropriate host components and facilitate systemic virus movement.