Biosynthesis of penicillin, cephalosporin C, and cephamycin C. Gene and organism names are in italics, names of enzymes are in capital letters. L-α-AAA is an intermediate of theL-lysine biosynthetic pathway but can also be provided by catabolic degradation of L-lysine. The question mark indicates that the contribution of the latter to β-lactam production has not been clarified yet. The penicillin biosynthesis occurs in fungi only, whereas cephalosporins are synthesized in both fungi (e.g., cephalosporin C by A. chrysogenum) and bacteria (e.g., cephamycin C by S. clavuligerus) (Fig. 1). See the text for details.