Phytotoxin production by Pseudomonas syringae pv syringae: Syringopeptin production by syr mutants defective in biosynthesis or secretion of syringomycin

Syringomycin and syringopeptin are lipodepsipeptide phytotoxins produced by Pseudomonas syringae pv. syringae. Four syr genes were identified previously and hypothesized to be involved in the regulation (syrA), biosynthesis (syrB and syrC), or export (syrD) of syringomycin. This study determines the influence of syr mutations on the composition of phytotoxic metabolites produced by P. syringae pv, syringae strain B301D-R. Levels of syringomycin and syringopeptin produced in liquid cultures were estimated by reverse phase HPLC analyses and differential antimicrobial assays. Significant quantities of syringopeptin were produced by both syrB and syrC mutants despite their inability to produce syringomycin. Only trace quantities of both lipodepsipeptides were produced by syrA and syrD mutants of P. syringae pv. syringae. These results indicate that syringomycin and syringopeptin an synthesized by separate pathways, but may share common mechanisms for secretion and regulation.