[2,3-'3C,]Pyruvic acid (7) was synthesized and administered to cultures of Pseudomonas syringae pv tubuci. C(2) and C(3) of 7 were incorporated as an intact unit into the plactam moiety of tabtoxin (1). The result suggests that the biosynthesis of 1 is proceeding in part along the lysine pathway. The labelling pattern in 1 and an incorporation experiment with a,a'-dideuterated (+)-2,6-diaminopimelic acid (19) indicate that the branching in the biosynthesis of 1 occurs before lysine is formed.Introduction. -Tabtoxin (Wildfire toxin; 1) is an extracellular pretoxin produced by the phytopathogenic bacterium Pseudomonas syringae pv tabaci. The biologically active plactam 1 is unstable [ 11. It is converted to the stable but biologically inactive &lactam isotabtoxin (2) by intramolecular transacylation. In our previous investigations on the biosynthesis of tabtoxin, three amino acids were found to be involved in forming the molecule [2]. Incorporation experiments with 13C-labelled compounds demonstrated that L-aspartate and L-threonine are precursors of the side chain, whereas the Me group of L-methionine was found to provide the carbonyl C-atom of the p-lactam moiety. Furthermore, it was shown that the remaining C, unit of the plactam moiety is not derived from acetate as e.g. in thienamycin [3], but from an intermediate originating from the C, pool (Scheme 1). In this paper, we wish to report the synthesis and results of the incorporation experiments of doubly 13C-labelled pyruvic acid 7 and a&-dideuterated diaminopimelic acid 18 (cf Scheme 4).