Many bacteria use nonribosomal peptide synthetase (NRPS) proteins to produce peptide antibiotics and siderophores. The catalytic domains of the NRPS proteins are usually linked in large multidomain proteins. Often, additional proteins are coexpressed with NRPS proteins that modify the NRPS peptide products, ensure the availability of substrate building blocks, or play a role in the import or export of the NRPS product. Many NRPS clusters include a small protein of ϳ80 amino acids with homology to the MbtH protein of mycobactin synthesis in Mycobacteria tuberculosis; no function has been assigned to these proteins. Pseudomonas aeruginosa utilizes an NRPS cluster to synthesize the siderophore pyoverdine. The pyoverdine peptide contains a dihydroxyquinoline-based chromophore, as well as two formyl-N-hydroxyornithine residues, which are involved in iron binding. The pyoverdine cluster contains four modular NRPS enzymes and 10 -15 additional proteins that are essential for pyoverdine production. Coexpressed with the pyoverdine synthetic enzymes is a 72-amino acid MbtH-like family member designated PA2412. We have determined the three-dimensional structure of the PA2412 protein and describe here the structure and the location of conserved regions. Additionally, we have further analyzed a deletion mutant of the PA2412 protein for growth and pyoverdine production. Our results demonstrate that PA2412 is necessary for the production or secretion of pyoverdine at normal levels. The PA2412 deletion strain is able to use exogenously produced pyoverdine, showing that there is no defect in the uptake or utilization of the iron-pyoverdine complex.Iron is an essential cofactor for many proteins, playing both catalytic and structural roles (1-3). Because of the low solubility of free Fe 3ϩ , many bacteria produce siderophores that bind to iron. The iron-siderophore complex is then actively transported into the cell. Although some small molecules like citrate or salicylate are able to function as lower affinity siderophores, many specialized peptides with extremely high affinity for iron are produced nonribosomally by bacteria. These compounds are produced by nonribosomal peptide synthetases (NRPSs) 2 that exist as modular proteins with multiple catalytic domains joined in a single protein. Often expressed with NRPS genes are other genes that encode proteins involved in additional essential steps including the synthesis of building blocks, siderophore export, import of the Fe 3ϩ -siderophore complex, or the removal Fe 3ϩ from the imported siderophore (4, 5). Pseudomonas aeruginosa is a bacterial pathogen that commonly causes infections in patients with cystic fibrosis (6). The siderophore pyoverdine (Fig. 1) is synthesized by P. aeruginosa (3) and has been correlated with virulence (7). Pyoverdine contains a cyclic peptide chain as well as a chemically modified dihydroxyquinoline-based chromophore that is responsible for iron binding. The peptide backbone of pyoverdine is synthesized by four large, multi-module NRPS proteins. The ...