Two coiled-coil regions of <Emphasis Type="Italic">Xanthomonas oryzae</Emphasis> pv. <Emphasis Type="Italic">oryzae</Emphasis> harpin differ in oligomerization and hypersensitive response induction

Hpa1_Xoo (harpin) is a type III secreted protein of the rice blight bacterial pathogen Xanthomonas oryzae pv. oryzae that elicits a hypersensitive response (HR) in nonhost tobacco. Hpa1_Xoo is predicted to contain two potential coiled-coil (CC) regions, one at the N-terminus with a high probability of formation, and one at the C-terminus with a lower probability of formation. We constructed several CC-equivalent peptides by a chemosynthetic method, and investigated the structure–function of the predicted Hpa1_Xoo CC regions, using biophysical and biochemical approaches. Both peptides elicited an HR in tobacco. Mutant versions of the N- and C-terminal peptides that were predicted to disrupt or favor CC formation were generated. The resulting altered HR activity and oligomerization indicated that the N-terminal CC region is essential for eliciting HR, but the C-terminus is not. The results also indicate that a 14-residue fragment (LDQLLCQLISALLQ) within the N-terminal CC region is a minimal and independent functional element for HR-induction in tobacco leaves. We propose that HR-induction requires a specific oligomerization of the CC regions of Hpa1_Xoo.