Although there is high overall sequence similarity between the polymyxin gene clusters of M-1, E681, and PKB1, the A domains in modules 6(X) and 7(Y) activate different amino acids. The identity between the amino acid sequences of the sixth modules of polymyxin synthetases of M-1 and E681, activating Phe and Leu, respectively, was only 88%. An even lower identity of 51% on the amino acid level was found for the A-domains of the seventh module in the polymyxin synthetases from M-1 and PKB1, activating either Thr or Leu, respectively. Polymyxin antibiotics are lipopeptides, GDC-0449 solubility dmso and as in case of the two other known pmx gene clusters, no genes

were found in the vicinity of the pmx gene cluster of P. polymyxa M-1 which might be involved in lipidation of the peptide moiety. It is likely that polymyxin synthesis resembles surfactin synthesis, and relies upon lipidation functions encoded elsewhere in the chromosome [32]. Notably, a thioesterase-like gene, pteH (COG3208), bearing a GrsT domain and similar to Bacillus amyloliquefaciens SrfAD (27% identity), was preceding a giant peptide synthetase gene at 2,508,313 in the genome of M-1. However, the PteH protein contains no acyltransferase domain and its role in attaching the fatty acid moiety to the polymyxin dekapeptide this website remains to be elusive. Discussion In this study, we found that growth of two important

phytopathogens, E. amylovora Ea273 and E. carotovora was inhibited by M-1. Polymyxin P was identified as being

the active principle of M-1. Two lines of evidence supported this finding: (1) M-1 supernatants formed a distinct clearing spot when exposed to bioautography using the Erwinia strains as indicator. When the material isolated from that area was analyzed by MALDI-TOF mass spectroscopy, GNE-0877 the mass peaks with m/z of 1199.9, 1213.9, 1253.9 and 1268.0 indicating alkali adducts of polymyxin P were detected (Figure 4); (2) a single fraction obtained by HPLC contained the inhibiting activity against bacterial pathogens and also the characteristic mass peaks m/z were indicating the presence of polymyxin P in this sample (Figure 5). Polymyxin P is a peptide antibiotic reported more than 40 years ago, and two species with different hydroxy fatty acids were described. Polymyxin P1 contains anteisononanoic acid, a-C9, and polymyxin P2 isooctanoic acid, i-C8[14]. Although its constituent amino acids have been determined as being six Dab, three Thr, and one Phe; to the best of our knowledge, no further investigation about the primary structure of polymyxin P and the configuration of the constituent amino acids has been performed until now. Here we established the primary structure of polymyxin P by PSD-MALDI-TOF mass spectrometry (Figure 3). Alterations in comparison to other polymyxin species were detected in two out of the four variable positions of the peptide.