1Department of Entomology and Plant Pathology, Horticulture Research International, East Malling, Kent ME19 6BJ, U.K.; 2Department of Biology, University of St. Andrews, Fife KY16 9ST, U.K.; 3Department of Virology, Institute of Microbiology, Beijing 100080, China; and 4Department of Disease and Stress Biology, John Innes Centre, Colney, Norwich NR4 7UH, U.K.
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Accepted 10 November 2001.
The nuclear localized C2 protein of the monopartite bego-movirus Tomato yellow leaf curl virus-China (TYLCV-C) contributes to viral pathogenicity. Here, we have investigated TYLCV-C C2 protein domains that play a role in the phenotype. Alignment of the C2 protein with 67 homologues from monopartite and bipartite begomoviruses re-vealed that a putative zinc-finger motif C36-X1-C38-X7-C46-X6-H53-X4-H58C59 and four potential phosphorylation sites(T52, S61, Y68, and S74) are highly conserved. When ex-pressed from a Potato virus X (PVX) vector, TYLCV-C C2 protein mutants C2-T52M, C2-H58S, C2-C59S, C2-S61R, and C2-S74D, like the wild-type C2 protein, induced local necrotic ringspots and systemic necrosis in Nicotiana ben-thamiana plants. Mutants C2-H53P and C2-Y68D produced irregular necrotic lesions on inoculated leaves that were distinct from the wild-type phenotype. In contrast, mutants C2-C36R, C2-C38N, and C2-C46I induced chlorosis and mosaic symptoms rather than necrosis. We demonstrate that TYLCV-C C2, like its counterpart in the bipartite begomovirus African cassava mosaic virus, mediates suppression of posttranscriptional gene silencing (PTGS). Moreover, the individual mutations C36R, C38N, and C46I abolished the ability of C2 protein to suppress PTGS. These results suggest that the three cysteine residues within the putative zinc-finger motif are essential for C2 protein to induce necrosis and to act as a suppressor of PTGS.
© 2002 The American Phytopathological Society