The virulence plasmid pJM1 enables the fish pathogen Vibrio anguillarum, a gram-negative polarly flagellated comma-shaped rod bacterium, to cause a highly fatal hemorrhagic septicemic disease in salmonids and other fishes, leading to epizootics throughout the world. The pJM1 plasmid 65,009-nucleotide sequence, with an overall G؉C content of 42.6%, revealed genes and open reading frames (ORFs) encoding iron transporters, nonribosomal peptide enzymes, and other proteins essential for the biosynthesis of the siderophore anguibactin. Of the 59 ORFs, approximately 32% were related to iron metabolic functions. The plasmid pJM1 confers on V. anguillarum the ability to take up ferric iron as a complex with anguibactin from a medium in which iron is chelated by transferrin, ethylenediamine-di(o-hydroxyphenyl-acetic acid), or other iron-chelating compounds. The fatDCBA-angRT operon as well as other downstream biosynthetic genes is bracketed by the homologous ISV-A1 and ISV-A2 insertion sequences. Other clusters on the plasmid also show an insertion element-flanked organization, including ORFs homologous to genes involved in the biosynthesis of 2,3-dihydroxybenzoic acid. Homologues of replication and partition genes are also identified on pJM1 adjacent to this region. ORFs with no known function represent approximately 30% of the pJM1 sequence. The insertion sequence elements in the composite transposon-like structures, corroborated by the G؉C content of the pJM1 sequence, suggest a modular composition of plasmid pJM1, biased towards acquisition of modules containing genes related to iron metabolic functions. We also show that there is considerable microheterogeneity in pJM1-like plasmids from virulent strains of V. anguillarum isolated from different geographical sources.The fish pathogen Vibrio anguillarum strain 775 is the causative agent of vibriosis, a highly fatal hemorrhagic septicemic disease (3). This bacterium disseminates in the vertebrate host by using the otherwise unavailable iron bound by high-affinity iron binding proteins, such as transferrin and lactoferrin. Furthermore, V. anguillarum 775 has the ability to grow in vitro in media in which iron is chelated by transferrin, ethylenediamine-di(o-hydroxyphenyl-acetic acid), and other iron chelators (15, 40). The metabolic pathway supporting the ability of this bacterium to grow under iron-limiting conditions is linked to the presence in the bacterial cells of the virulence plasmid pJM1 (15).Iron metabolic plasmids are rare; in addition to the pJM1-like plasmids only the pColV-K30 family of plasmids, identified in human clinical strains of Escherichia coli and other enteric bacteria, have been associated with iron metabolism. However, the pJM1 and pColV-K30 plasmid-mediated iron uptake systems are unrelated (17, 49). pJM1-like-plasmids, usually around 65 to 67 kbp, have been reported by us and others in different virulent V. anguillarum strains isolated from many epizootics throughout the world (30,32,46). In this family of plasmids the best characterized is t...