SYRINGOMYCIN PRODUCTION AMONG STRAINS OF PSEUDOMONAS-SYRINGAE PV SYRINGAE - CONSERVATION OF THE SYRB AND SYRD GENES AND ACTIVATION OF PHYTOTOXIN PRODUCTION BY PLANT SIGNAL MOLECULES

The syrB and syrD genes of Pseudomonas syringae pv. syringae are predi cted to encode proteins that function in the synthesis and export of s yringomycin, respectively. Using portions of the syr genes as DNA prob es, both genes were shown to be conserved as single copies within a 15 -kb or smaller DNA region among a broad spectrum of P. s. pv. syringae strains that produce syringomycin or one of its amino acid analogs, s yringotoxin and syringostatin. Strains representative of P. viridiflav a and six pathovars of P. syringae failed to hybridize with the gene p robes, demonstrating that syr sequences are highly specific to P. s. p v. syringae and related nonpathogenic strains. Maximum parsimony analy sis of restriction fragment length polymorphism profiles was used to e valuate relatedness among strains within the syrB and syrD gene region . A tree, conveying the smallest number of evolutionary changes among strains, revealed considerable diversity within the syr gene region; s ubclusters of strains were identified that appear to share specific qu alities relevant to the plant-pathogen interaction. Because both the s yrB gene and syringomycin production can be induced in response to pla nt signal molecules, 42 strains containing homologous syr sequences we re tested for signal-mediated induction of toxin production. Over 90% of the toxigenic strains produced larger quantities of toxin when the plant signal molecules, arbutin and D-fructose, were added to syringom ycin-minimal medium; 13 of the strains produced greater than or equal to 10-fold higher toxin levels. Some strains, such as 5D428, produced toxin only in the presence of these signals. This demonstrates that ne arly all strains of P. s. pv. syringae have a sensory mechanism for sp ecific plant metabolites that modulate syringomycin, syringotoxin, or syringostatin production.