S-adenosyl-L-methionine-dependent methyltransferase involved in the formation of 3-methylarginine (MeArg), a potent antibiotic against the closely related soybean pathogen P.syringae pv. glycinea. Transfers the methyl group from S-adenosyl-L-methionine into 5-guanidino-2-oxopentanoate acid to yield 5-guanidino-3-methyl-2-oxopentanoate5-guanidino-3-methyl-2-oxopentanoate, a precursor of MeArg. 5-guanidino-2-oxopentanoate + S-adenosyl-L-methionine = 5-guanidino-3-methyl-2-oxopentanoate + H(+) + S-adenosyl-L-homocysteine kcat is 85 min(-1). Antibiotic biosynthesis.