Biosynthesis of the β-Methylarginine Residue of Peptidyl Nucleoside Arginomycin in Streptomyces arginensis NRRL 15941

The peptidyl nucleoside arginomycin is active against Gram-positive bacteria and fungi but displays much lower toxicity to mice than its analog blasticidin S. It features a rare amino acid, beta-methylarginine, which is attached to the deoxyhexose moiety via a 4'-aminoacyl bond. We here report cloning of the complete biosynthetic gene cluster for arginomycin from Streptomyces arginensis NRRL 15941. Among the 14 putative essential open reading frames, argM, encoding an aspartate aminotransferase (AAT), and adjacent argN, encoding an S-adenosyl methionine (SAM)-dependent methyltransferase, are coupled to catalyze arginine and yield beta-methylarginine in Escherichia coli. Purified ArgM can transfer the alpha-amino group of L-arginine to alpha-ketoglutaric acid to give glutamate and thereby converts L-arginine to 5-guanidino-2-oxopentanoic acid, which is methylated at the C-3 position by ArgN to form 5-guanidino-3-methyl-2-oxopentanoic acid. Iteratively, ArgM specifically catalyzes transamination from the donor L-aspartate to the resulting 5-guanidino-3-methyl-2-oxopentanoic acid, generating beta-methylarginine. The complete and concise biosynthetic pathway for the rare and bioactive amino acid revealed by this study may pave the way for the production of beta-methylarginine either by enzymatic conversion or by engineered living cells.