Characterization of the thiotemplate mechanisms of syringomycin and syringopeptin synthesis by Pseudomonas syringae pv. syringae

A distinctive feature of Pseudomonas syringae pv. syringae is the production of two lipopeptide toxins, syringomycin and syringopeptin, which form pores in plasma membranes and cause passive transmembrane ion fluxes. Syringomycin and syringopeptin are the major virulence determinants of P. s. pv. syringae based on pathogenicity tests of syr and syp biosynthesis mutants. The biosynthesis of both phytotoxins is predicted to occur on multifunctional enzyme complexes by the thiotemplate mechanism. The syringomycin (syr) gene cluster lies adjacent to the syringopeptin (syp) gene cluster on the P. s. pv. syringae chromosome, and together they compose over 1% of the genome. A model for syringomycin biosynthesis and its underlying genetic organization is presented. Most of the similar to 35 kb syr gene cluster encodes Syr synthetases composed of amino acid-activating domains that catalyze the adenylation of the constituent amino acids and thioester formation. The role of phenolic plant signal molecules in the activation of syrB gene expression and syringomycin production is summarized. This represents the first example of a nonribosomal peptide synthetase gene being activated by specific plant-derived signal molecules.