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A cloned avirulence gene from Pseudomonas solanacearum determines incompatibility on Nicotiana tabacum at the host species level

A locus in Pseudomonas solanacearum AW1 responsible for the hypersensitive response (HR) on tobacco was cloned by complementation in the tobacco-pathogenic strain P. solanacearum NC252. The NC252 HR+ transconjugants lost pathogenicity on tobacco, indicating that the cloned locus could restrict the host range of NC252. Restriction enzyme mapping, transposon mutagenesis, and subcloning showed that, at most, 2.0 kilobases of the cloned DNA was required for NC252 transconjugants to elicit HR on tobacco. Site-directed insertional mutagenesis of the wild-type locus in strain AW1 to create AW1-31 eliminated HR activity on tobacco. However, AW1-31 retained pathogenicity on tomato and eggplant, confirming that this locus contains an avirulence gene, designated avrA. In contrast to the wild type, AW1-31 multiplied to almost the same extent as NC252 after infiltration into tobacco leaves. Nevertheless, AW1-31 did not wilt tobacco when stem inoculated, suggesting that additional factors condition host range. AW1 was HR+ on 27 N. tabacum cultivars, whereas AW1-31 was always HR-, strongly suggesting that avrA is specific at the host species level.