Figure 7 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 AP26113 cost transporter-like genes. The position of the gene cluster within the chromosome of M-1 is indicated. (B) Domain organization of the Selleck BMN 673 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 C646 [28], while Thr at the seventh position (Y) of polymyxin P is substituted by Leu at the corresponding position

of polymyxin B [32]. Polymyxin A and polymyxin B are labelled as “PA” and “PB”, respectively. Domain analysis performed with the NRPSpredictor2 server of the university of Tuebingen [43] revealed that the putative polymyxin synthetase of M-1 comprises ten modules (Figure 7B). Each of them consists of three or four domains, such as A-T-C, A-T-E-C or A-T-TE. However, similar to the pmx gene clusters in P. polymyxa PKB1 and P. polymyxa E681, the order and arrangement of the NRPS encoding genes was not collinear with the amino acids in the polymyxin end product. PmxA, a polypeptide containing 5010 amino acids, comprised four modules. The substrate specificities of the four adenylation Rutecarpine domains (A-domain) were predicted to activate the amino acid substrates D-Phe-6, L-Thr-7, L-Dab-8 and L-Dab-9, respectively. PmxB, a polypeptide consisting of 1102 amino acids, contained the remaining part of the last module including a thioesterase domain (TE-domain), A-T-TE. The A-domain was predicted to activate L-Thr-10. PmxE, a 6312 amino-acid polypeptide, contained five modules responsible for the first five amino acids of polymyxin P. In addition, a N-terminal condensation

domain with similarity to starter C-domain simultaneously acylating the first amino acid with a fatty acid tail was identified [44]. The five A-domains were predicted to activate L-Dab-1, L-Thr-2, D-Dab-3, L-Dab-4, and L-Dab-5, respectively. Therefore, the ten modules were arranged in the gene order pmxE-pmxA-pmxB (Figure 7B). There were two epimerization domains (E-domains), occurring in the third and sixth module, which indicated that the third and sixth amino acid of the polymyxin produced by M-1 represented D-forms, D-Dab and D-Phe, respectively. The TE-domain located at the carboxy-terminal region of PmxB was probably responsible for terminating polymyxin synthesis by cyclization and releasing the product.