Putative gene cluster for polymyxin biosynthesis in P. polymyxa M-1 and primary structure of polymyxin P. (A) Genetic structure of the pmx genes. Black filled arrows represent NRPS genes, while white arrows represent ABC transporter-like genes. The position of the gene cluster within the chromosome of M-1 is indicated. (B) Domain organization of the putative Pmx enzymes. (C) Primary structure of polymyxin P synthesized in P. polymyxa M-1 derived by bioinformatic and chemical analysis. FA, fatty acid, 6-methyloctanoic acid or isooctanoic acid. “1-10” indicate the ten amino acid moieties. Four variable sites were marked as “W, X, Y and Z”, respectively. Phe at the sixth position (X) of polymyxin P is replaced by Leu at the corresponding position of polymyxin A , while Thr at the seventh position (Y) of polymyxin P is substituted by Leu at the corresponding position of polymyxin B . Polymyxin A and polymyxin B are labelled as “PA” and “PB”, respectively.