SUPPRESSION OF POTYVIRUS INFECTION BY COEXPRESSED CLOSTEROVIRUS PROTEIN

A tobacco etch virus (TEV)-based expression vector has been used for i nsertion of several ORFs derived from the unrelated beet yellows virus (BW). Hybrid TEV variants expressing the BW capsid protein, 20-kDa pr otein, or HSP70 homolog systemically infected Nicotiana tabacum and st ably retained BW sequences. In contrast, insertion of the ORF encoding BW leader proteinase (L-Pro) resulted in severely impaired systemic t ransport and accumulation of recombinant TEV. Progeny of this virus un derwent various deletions affecting the L-Pro sequence and mitigating the defects in virus spread. Model experiments involving several spont aneous and engineered mutants indicated that the central domain of BW L-Pro was responsible for the defect in hybrid virus accumulation, whe reas full-size L-Pro was required for maximal debilitation of systemic transport. Strikingly, BW L-Pro expression did not debilitate systemi c infection of hybrid TEV in Nicotiana benthamiana plants. No major de fects in replication or encapsidation of recombinant RNA were revealed in N. tabacum protoplasts. These results indicated that BW L-Pro spec ifically interfered with TEV systemic transport and accumulation in a host-dependent manner and suggested a potential utility of closterovir us L-Pro as an inhibitor of potyvirus infection. In addition, it was d emonstrated that the 107-amino-acid-residues-long N-terminal part of t he TEV helper component proteinase is not essential for systemic infec tion. (C) 1997 Academic Press.