The epiphyte Pseudomonas syringae pv. syringae 22d/93 (Pss22d) produces the rare amino acid 3-methylarginine (MeArg), which is highly active against the closely related soybean pathogen Pseudomonas syringae pv. glycinea. Since these pathogens compete for the same habitat, Pss22d is a promising candidate for biocontrol of P. syringae pv. glycinea. The MeArg biosynthesis gene cluster codes for the S-adenosylmethionine (SAM)-dependent methyltransferase MrsA, the putative aminotransferase MrsB, and the amino acid exporter MrsC. Transfer of the whole gene cluster into Escherichia coli resulted in heterologous production of MeArg. The methyltransferase MrsA was overexpressed in E. coli as a His-tagged protein and functionally characterized (K m , 7 mM; k cat , 85 min ؊1 ). The highly selective methyltransferase MrsA transfers the methyl group from SAM into 5-guanidino-2-oxo-pentanoic acid to yield 5-guanidino-3-methyl-2-oxo-pentanoic acid, which then only needs to be transaminated to result in the antibiotic MeArg.Microbial plant pathogens cause severe losses in agriculture each year (1). For example, the plant pathogen Pseudomonas syringae pv. glycinea is responsible for bacterial blight of soybean, a leaf spot disease of great economic impact. Besides chemical treatment, biocontrol agents that antagonize microbial plant pathogens are gaining increasing importance in fighting plant diseases (6,11,27). In a screening for possible biocontrol strains, an epiphytic bacterium showing a strong and selective activity against the pathogen P. syringae pv. glycinea was isolated from soybean leaves (29). The strain was characterized as Pseudomonas syringae pv. syringae 22d/93 (Pss22d). The antagonism of Pss22d against P. syringae pv. glycinea has been demonstrated successfully in vitro and in planta under greenhouse and field conditions (19,29). In order to identify the molecular basis of the antagonism of Pss22d against P. syringae pv. glycinea, we focused on its secondary metabolites. Besides the well-known lipodepsipeptides syringomycin and syringopeptin (3), Pss22d produces the rare amino acid 3-methylarginine (MeArg) (5). As little as 20 nmol of MeArg strongly and selectively inhibits P. syringae pv. glycinea but no other pseudomonads in vitro (29). Since the inhibition can be compensated for by L-arginine supplementation but not by any other essential amino acid, it is likely that the toxin acts as an inhibitor of the arginine biosynthesis pathway or an argininedependent pathway, such as nitric oxide formation (13,16). Feeding experiments and Tn5 transposon mutagenesis suggested that MeArg is produced by an S-adenosyl methionine (SAM)-dependent methyltransferase (5) converting the enol of 5-guanidino-2-oxo-pentanoic acid to 5-guanidino-3-methyl-2-oxo-pentanoic acid. An analogous reaction is known to occur with the methyltransferases GlmT, DptI, and LptI, which form 3-methylglutamate from ␣-ketoglutarate (18). On the way to MeArg, only a transaminase catalyzing the formation of MeArg from 5-guanidino-3-methyl-2-oxo-pentano...