Cloned avirulence gene of Pseudomonas syringae pv. glycinea determines race-specific incompatibility on Glycine max (L.) Merr

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Abstract
A genomic library of P. syringae pathovar glycinea race 6 DNA was constructed in the mobilizable cosmid vector pLAFR1 and maintained in Escherichia coli HB101. Completeness of the library was estimated by assaying clones for the expression of ice-nucleating activity in E. coli. Ice-nucleation activity was represented .apprx. once in every 600 clones. Six hundred eighty random race 6 cosmid clones were mobilized from E. coli by plasmid pRK2013 in individual conjugations to a race 5 strain of P. syringae glycinea. A single clone (pPg6L3) was detected that changed the race specificity of race 5 from virulent (compatible) to avirulent (incompatible) on the appropriate soybean cultivars. The clone was also mobilized from E. coli into race 1 and race 4 strains of P. syringae glycinea, and it conferred on these transconjugants the same host range incompatibility as the wild-type race 6 strain. The cosmid clone was mapped by restriction endonucleases, and 2 adjacent EcoRI fragments were identified by transposon Tn5 mutagenesis to be important in determining race specificity. Southern blot analysis showed that the 2 EcoRI fragments are unique to race 6 and are not present in the other races tested. The cosmid clone pPg6L3 was also mobilized to P. fluorescens and Rhizobium japonicum. Neither these isolates nor E. coli harboring pPg6L3 elicited a hypersensitive reaction in soybean leaves.