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Structural and Functional Conservation of the Tobamovirus Coat Protein Elicitor Active Site

Elicitation of the N′ gene hypersensitive response (HR) in Nicotiana sylvestris is dependent upon specific structural elements of an elicitor active site located along the right face of the tobacco mosaic tobamovirus (TMV) coat protein (CP) (Z. Taraporewala and J. N. Culver, Plant Cell 8:169-178, 1996). In this study, the functional and structural conservation of this site was examined in three different tobamovirus CPs that vary widely in sequence homology but share a common three-dimensional fold. Results demonstrate that the CPs of TMV-U2, Odontoglossum ringspot, and cucumber green mottle mosaic tobamoviruses elicit the N′ gene HR with strength and temperature stability comparable to the TMV elicitor CP P20L. Within each CP, amino acid substitutions designed to disrupt tertiary structure interfered with N′ gene elicitation. Substitutions of selected surface residues within the previously identified elicitor active site of each CP inhibited HR elicitation, while substitutions outside of this region did not alter elicitor activity. Comparisons of surface characteristics within the elicitor active sites of the different CPs revealed the presence of a conserved central hydrophobic cavity. Surface features at the periphery of this cavity showed less conservation, which may explain the unequal ability of some substitutions in this region to alter the elicitor activity of the different CPs. These findings suggest N′ gene specificity is dependent upon the three dimensional fold of CP as well as upon specific surface features within the elicitor active site.