Anguibactin, a siderophore produced by Vibrio anguillarum, is synthesized via a nonribosomal peptide synthetase (NRPS) mechanism. We have identified a gene from the V. anguillarum plasmid pJM1 that encodes a 78-kDa NRPS protein termed AngM, which is essential in the biosynthesis of anguibactin. The predicted AngM amino acid sequence shows regions of homology to the consensus sequence for the peptidyl carrier protein (PCP) and the condensation (C) domains of NRPSs, and curiously, these two domains are not associated with an adenylation (A) domain. Substitution by alanine of the serine 215 in the PCP domain and of histidine 406 in the C domain of AngM results in an anguibactin-deficient phenotype, underscoring the importance of these two domains in the function of this protein. The mutations in angM that affected anguibactin production also resulted in a dramatic attenuation of the virulence of V. anguillarum 775, highlighting the importance of this gene in the establishment of a septicemic infection in the vertebrate host. Transcription of the angM gene is initiated at an upstream transposase gene promoter that is repressed by the Fur protein in the presence of iron. Analysis of the sequence at this promoter showed that it overlaps the iron transport-biosynthesis promoter and operates in the opposite direction.