Legionella pneumophila, a parasite of alveolar macrophages, requires iron for intra- and extracellular growth. Although its mechanisms for iron assimilation are poorly understood, this bacterium produces Fur, a protein that can repress gene transcription in response to iron concentration. Because iron- and Fur-regulated genes are important for infection in other bacteria, the identification of similar genes in L. pneumophila was undertaken. A wild-type strain of L. pneumophila was randomly mutated with a mini-Tn10' lacZ transposon, and the resulting gene fusions were tested for iron regulation by assessing beta-galactosidase production in the presence and absence of iron chelators. Of the initial six mutants with iron-repressed lacZ fusions, two strains, NU229 and NU232, possessed fusions that were stably iron regulated. To assay for Fur regulation, the levels of beta-galactosidase were measured in strains no longer producing Fur. As in a number of pathogenic bacteria, L. pneumophila fur could not be insertionally inactivated, but spontaneous Fur- derivatives were generated by selecting for manganese resistance. Strain NU229 contained a Fur-repressed fusion based on derepression of lacZ expression in its manganese-resistant derivative. Extracellular growth of NU229 in bacteriological media was similar to that of wild-type strain 130b. To assess the role of an iron- and Fur-regulated (frgA) gene in intracellular infection, the ability of NU229 to grow within U937 cell monolayers was tested. Quantitative infection assays demonstrated that intracellular growth of NU229 was impaired as much as 80-fold. Reconstruction of the mutant by allelic exchange proved that the infectivity defect in NU229 was due to the inactivation of frgA and not to a second-site mutation. Subsequently, complementation of the interrupted gene by an intact plasmid-encoded gene demonstrated that the infectivity defect was due to the loss of frgA and not to a polar effect. Nucleotide sequence analysis revealed that the 63-kDa FrgA protein has homology with the aerobactin synthetases IucA and IucC of Escherichia coli, raising the possibility that L. pneumophila encodes a siderophore which is required for optimal intracellular replication. Southern hybridization analysis determined that frgA is specific to L. pneumophila.