Discovery and biosynthesis of gladiochelins: unusual lipodepsipeptide siderophores from Burkholderia gladioli
Burkholderia is a genus of diverse Gram-negative bacteria that includes several opportunistic pathogens. Siderophores, which transport iron from the environment into microbial cells, are important virulence factors in most pathogenic Burkholderia species. However, it is widely believed that Burkholderia gladioli, which can infect the lungs of cystic fibrosis (CF) sufferers, does not produce siderophores. B. gladioli BCC0238, isolated from the lung of a CF patient, produces two novel metabolites in a minimal medium containing glycerol and ribose as carbon sources. HPLC purification, followed by detailed spectroscopic analyses, identified these metabolites as unusual lipodepsipeptides containing a unique citrate-derived fatty acid and a rare dehydro-{beta}-alanine residue. The absolute configurations of the amino acid residues in the two metabolites was elucidated using Marfeys method and the gene cluster responsible for their biosynthesis was identified by bioinformatics and insertional mutagenesis. In-frame deletions and enzyme activity assays were used to investigate the functions of several proteins encoded by the biosynthetic gene cluster, which was found in the genomes of most B. gladioli isolates, suggesting that its metabolic products play an important role in the growth and/or survival of the species. The Chrome Azurol S (CAS) assay showed the metabolites bind ferric iron and that this supresses their production when added to the growth medium. Moreover, a gene encoding a TonB-dependent ferric-siderophore receptor is adjacent to the biosynthetic genes. Together, these observations suggest that these metabolites likely function as siderophores in B. gladioli.