The dual function in virulence and host range restriction of a gene isolated from the pPATH (Ehg) plasmid of Erwinia herbicola pv. gypsophilae

The host range of the gall-forming bacterium Erwinia herbicola pv. gypsophilae (Ehg) is restricted to gypsophila whereas Erwinia herbicola pv. betae (Ehb) attacks beet as well as gypsophila. Both pathovars contain an indigenous plasmid (pPATH(Ehg or pPATH(Ehb)) that harbors pathogenicity genes, including the hrp gene cluster. A cosmid library of Ehg824-1 plasmid DNA was mobilized into Ehb4188 and the transconjugants were screened for pathogenicity on beet. One Ehb transconjugant harboring the cosmid pLA173 of pPATHEb induced a hypersensitive-like response and abolished pathogenicity on beet. Transposon mutagenesis of an open reading frame (ORF) located on this cosmid eliminated its affect on pathogenicity. Marker exchange of this mutation into Ehg824-1 caused a substantial reduction in gall size on gypsophila and caused Ehg824-1 to extend its host range and incite galls on beet. The ORF (1.5 kb) was designated as pthG (pathogenicity gene on gypsophila). DNA sequence analysis of pthG revealed no significant homology to known genes in the data bank. Only remnants of the pthG sequences were identified on the pPATH of Ehb4188. The deduced protein lacked an N-terminal signal peptide but contained a short trans-membrane helix in its C terminus. The gene product, as determined by expression in Escherichia coli and Western blots (immunoblots), was a 56-kDa protein.