The Pseudomonas aeruginosa fabI structural gene, encoding enoyl-acyl carrier protein (ACP) reductase, was cloned and sequenced. Nucleotide sequence analysis revealed that fabIis probably the last gene in a transcriptional unit that includes a gene encoding an ATP-binding protein of an ABC transporter of unknown function. The FabI protein was similar in size and primary sequence to other bacterial enoyl-ACP reductases, and it contained signature motifs for the FAD-dependent pyridine nucleotide reductase and glucose/ribitol dehydrogenase families, respectively. The chromosomal fabIgene was disrupted, and the resulting mutant was viable but possessed only 62% of the total enoyl-ACP reductase activity found in wild-type cell extracts. The fabI-encoded enoyl-ACP reductase activity was NADH dependent and inhibited by triclosan; the residual activity in the fabI mutant was also NADH dependent but not inhibited by triclosan. An polyhistidine-tagged FabI protein was purified and characterized. Purified FabI (i) could use NADH but not NADPH as a cofactor; (ii) used both crotonyl-coenzyme A and crotonyl-ACP as substrates, although it was sixfold more active with crotonyl-ACP; and (iii) was efficiently inhibited by low concentrations of triclosan. A FabI Gly⁹⁵-to-Val active-site amino acid substitution was generated by site-directed mutagenesis, and the mutant protein was purified. The mutant FabI protein retained normal enoyl-ACP reductase activity but was highly triclosan resistant. When coupled to FabI, purified P. aeruginosa N-butyryl-l-homoserine lactone (C₄-HSL) synthase, RhlI, could synthesize C₄-HSL from crotonyl-ACP and S-adenosylmethionine. This reaction was NADH dependent and inhibited by triclosan. The levels of C₄-HSL andN-(3-oxo)-dodecanoyl-l-homoserine lactones were reduced 50% in a fabI mutant, corroborating the role of FabI in acylated homoserine lactone synthesis in vivo.