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, gypsophi lae (Ehg) is restricted to gypsophila whereas Erwinia herbicola pv. betae ( Ehb) attacks beet as well as gypsophila. Both pathovars contain an indigeno us plasmid (pPATH(Ehg) or pPATH(Ehb)) that harbors pathogenicity genes, inc luding the hrp gene cluster. A cosmid library of Ehg824-1 plasmid DNA was m obilized into Ehb4188 and the transconjugants were screened for pathogenici ty on beet. One Ehb transconjugant harboring the cosmid pLA173 of pPATH-(Eh g), induced a hypersensitivelike response and abolished pathogenicity on be et. Transposon mutagenesis of an open reading frame (ORF) located on this c osmid eliminated its affect on pathogenicity. Marker exchange of this mutat ion into Ehg823-1 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). DN A sequence analysis of pthG revealed no significant homology to known genes in the data bank. Only remnants of the pthG sequences were identified on t he pPATH of Ehb4188. The deduced protein lacked an N-terminal signal peptid e but contained a short trans-membrane helix in its C terminus. The gene pr oduct, as determined by expression in Escherichia call and Western blots (i mmunoblots), was a 56-kDa protein.