MUTATIONS IN THE REGION ENCODING THE CENTRAL DOMAIN OF HELPER COMPONENT-PROTEINASE (HC-PRO) ELIMINATE POTATO-VIRUS X POTYVIRAL SYNERGISM/

Coinfection of tobacco plants with potato virus X (PVX) and any of sev eral members of the potyvirus group causes a synergistic disease chara cterized by a dramatic increase in symptom severity correlated with a 3- to 10-fold increase in the accumulation of PVX in the first systemi cally infected leaves. We have recently shown that PVX/potyviral syner gistic disease is mediated by expression of potyviral 5'-proximal sequ ences encoding P1, helper component-proteinase (HC-Pro), and a fractio n of P3 (termed P1/HC-Pro sequence). Here we report the effect of muta tions in this potyviral sequence on the induction or synergistic disea se. Three transgenic tobacco lines expressing the tobacco etch potyvir us (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 i n the HC-Pro coding region were unable to induce the synergistic incre ases in either symptom severity or PVX accumulation. Loss of synergist ic function was associated with mutations within the region encoding t he 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 experimen ts, a TEV mutant lacking the sequence encoding the zinc-finger domain of HC-Pro induced a typical synergistic response in interaction with P VX. The results indicate that the zinc-finger domain comprising the fi rst 66 amino acid residues of HC-Pro is dispensable for induction of s ynergistic disease and transactivation of PVX multiplication, while re gions within the central domain of HC-Pro are essential for both of th ese responses. (C) 1997 Academic Press.