Biosynthesis of Monobactam Compounds: Origin of the Carbon Atoms in the Beta-lactam Ring

The biosynthesis of monobactams by strains of Chromobacterium violaceum, Acetobacter sp., and Agrobacterium radiobacter was studied. Monobactams were produced during logarithmic growth by C. violaceum and Acetobacter sp. and during late log growth on glycerol and in stationary phase by A. radiobacter. The addition of various amino acids failed to significantly stimulate monobactam production in any of the producing organisms. Several 14C-amino acids and pyruvate were incorporated in vivo into monobactams. Serine, glycine, and cysteine were better incorporated than alanine or aspartate, whereas an excess of nonradioactive serine depressed the incorporation of labelled cysteine, glycine, and pyruvate. A comparison of [1-14C] glycine and [2-14C] glycine incorporation data suggests that glycine was first converted to serine. With a mixture of [U-14C[serine and [3-3H]serine, C. violaceum synthesized a monobactam with a complete retention of tritium, whereas with a [U-14C] cystine and [3-3H] cystine mixture, there was an extensive loss of C-3 tritium. Acetobacter sp. and A. radiobacter also utilized the double-labeled serine without the loss of tritium in their respective monobactams. It appears, therefore that in the three organisms, the carbon atoms of the beta-lactam ring of the monobactam are derived directly from serine without the loss of the C-3 hydrogen atoms, probably by an SN2 ring closure mechanism. With [methyl-14C] methionine, most of the radioactivity in the monobactam from Acetobacter sp. was in the methyl moiety of the beta-lactam ring methoxyl group.