Mangotoxin:: a novel antimetabolite toxin produced by Pseudomonas syringae inhibiting ornithine/arginine biosynthesis

Pseudomonas syringae pv.. syringae is a common inhabitant of a large variety of plants and the Causal agent of bacterial apical necrosis of mango. In this work, we describe production and preliminary characterization of a novel anti metabolite toxin, designated mangotoxin, by P. syringae pv. syringae strains mainly isolated from mango trees. The toxin was detected by the Escherichia coli growth inhibition assay, since growth inhibition was reversed by L-ornithine and not by N-acetyl ornithine, Suggesting that mangotoxin could interfere with this step of ornithine/arginine biosynthesis. Cell-free culture filtrates from different mangotoxin-producing strains strongly decreased, around 75%, the activity of ornithine N-acetyltransferase (OAT; EC 2.3.1.35) from tomato leaf protein extracts, confirming this enzyme as a target of the novel toxin. Biochemical data suggest that mangotoxin is a hydrophylic oligopeptide (< 3 kDa) without a complex secondary structure (sensitive to proteases, but stable at high temperatures and extreme pH), as other anti metabolite toxins, such as phaseolotoxin or tabtoxin. HPLC analyses of cell-free culture filtrates allowed us to associate mangotoxin activity to a single peak obtained from a mangotoxin-producer strain but not present in filtrates from a derivative mutant defective in the toxic activity. (C) 2003 Elsevier Ltd. All rights reserved.