IDENTIFICATION OF AN ELICITOR-ACTIVE SITE WITHIN THE 3-DIMENSIONAL STRUCTURE OF THE TOBACCO MOSAIC TOBAMOVIRUS COAT PROTEIN

The coat protein (CP) of tobacco mosaic tobamovirus (TMV) elicits the hypersensitive response (HR) conferred by the N' gene from Nicotiana s ylvestris. This study presents evidence demonstrating a critical role for a specific CP structural site in eliciting this HR. Based on the k nown structure of the TMV CP, specific substitutions were created with in the CP of the elicitor strain P20L to identify structural areas ess ential for host recognition. Of 32 substitutions made, 14 conferred ei ther a temperature-sensitive (loss of the HR at 29 degrees C) or a kno ckout (loss of the HR at 25 degrees C) HR phenotype in N. sylvestris. These essential residues were noncontiguous in position; however, with in the three-dimensional CP structure, all resided primarily along the right face of the molecule's helical bundle. Substitutions that did n ot affect the HR phenotype either were located outside of this area or were conservative in change. In addition, placing two temperature-sen sitive substitutions within the same CP resulted in lowering temperatu re sensitivity from 29 to 27 degrees C. This additive effect suggests that residues essential for HR elicitation contribute independently to host recognition. This feature is characteristic of recognition surfa ces. The presence of a specific elicitor active site within the three- dimensional structure of the TMV CP is consistent with binding of a ho st-encoded receptor and demonstrates the importance of CP structure in HR specificity.