Coinfection of tobacco plants with potato virus X (PVX) and any of several members of the potyvirus group causes a synergistic disease characterized by a dramatic increase in symptom severity correlated with a 3- to 10-fold increase in the accumulation of PVX in the first systemically infected leaves. We have recently shown that PVX/potyviral synergistic disease is mediated by expression of potyviral 5'-proximal sequences encoding P1, helper component-proteinase (HC-Pro), and a fraction of P3 (termed P1/HC-Pro sequence). Here we report the effect of mutations in this potyviral sequence on the induction of synergistic disease. Three transgenic tobacco lines expressing the tobacco etch potyvirus (TEV) P1/HC-Pro sequence with mutations within the P1 coding region were not impaired in their ability to mediate synergism when infected with PVX. In contrast, two of three transgenic lines with mutations in the HC-Pro coding region were unable to induce the synergistic increases in either symptom severity or PVX accumulation. Loss of synergistic function was associated with mutations within the region encoding the central domain of HC-Pro, while the ability to induce synergism was retained in a transgenic line expressing HC-Pro with an alteration in the amino-terminal 'zinc-finger domain'. In coinoculation experiments, a TEV mutant lacking the sequence encoding the zinc-finger domain of HC-Pro induced a typical synergistic response in interaction with PVX. The results indicate that the zinc-finger domain comprising the first 66 amino acid residues of HC-Pro is dispensable for induction of synergistic disease and transactivation of PVX multiplication, while regions within the central domain of HC-Pro are essential for both of these responses.
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