PmxA synthetase involved in polymyxin E synthesis, comprising four adenylation sites, characterized in that the second adenylation site has at least 90% identity with the peptide sequence SEQ ID NO 1: VTEAEKADLLGRFNDTTTEFPRGKTLIQLFEEQVERIPDAAAITLNEQE LTYRELNERVNRLARTLRSHGISKGRLVAILAERSIEMVVGMLAAHKAG AAYVPIDPEYPEERIRFLIEDSGGQVMLTQSRLRERLAGSDPVILLDDE SFYHEDGTNLNTGIEATDLACVIYTSGTTGKPKGNPVSHRNIVRVVQNT NYIDITERDHVLQLSSYSFDGATFDIFGALTNGARLVLVPYETLLEIGR LADLIQRERISVMFITTAFFNILVDVNVDCLRDVRAILFGGERVSVGHV RKALAHIGPGRLNHVYGPTESTVYTTYLPVDFVDELAVTVPIGRPISNT TVYIVDSRNKLLPIGVAGELCVGGEGLVRGYNNRPELTAEKFVDNPFVP GERMYRTGDLAKWLPDGTIEYVGRTDDQVKIRGFRIELGEIEAQLQKVE GIRKTTVFARENASGEKQLCAYYEADCELPAAELKSVLSKELPAYMIPA YLIQLERLPLTTNGKVDRRSLPAPEESLQPGGG, the underlined amino acids DGFFLGVVYK being conserved and forming a binding pocket specific for a leucine, isoleucine or valine residue.