In Escherichia coli at least five enzyme activities required for the beta-oxidation of fatty acids are associated with a multienzyme complex composed of two subunits in &2182 conformation (A. Pramanik et al., J. Bacteriol. 137:469-473, 1979). In the present work, the DNA sequence of the genes encoding these two subunits,fadB and fadA, has been determined. The direction of transcription was from fadB to fadA rather than from fadA to fadB, as suggested previously (S. K. Spratt et al., J. Bacteriol. 158:535-542, 1984). Only 10 nucleotides separated the coding sequences for the two peptides, confirming the suggestion that these genes form an operon. The peptides encoded byfadB andfadA were 729 amino acids and 387 amino acids, respectively, in length. The larger and smaller peptides had predicted molecular masses of 79,678 and 40,876 Da, respectively. Recently, the sequence of thefadA gene was published in a separate report (Yang et al., J. Biol. Chem. 265:10424-10429, 1990). In this work, most of the DNA sequence forfad4 was confirmed, and 10 errors were corrected. Three of these nucleotide changes resulted in five amino acid residue changes predicted in the carboxy terminus of the fadA-encoded peptide. By comparison to other peptide sequences, the a subunit encoded withinfadB had 31% perfect identity with the rat peroxisomal enoyl-coenzyme A:hydratase-3-hydroxyacyl-coenzyme A dehydrogenase trifunctional enzyme over the entire length of the two peptides. In agreement with the work of Yang et al., the , subunit encoded within fadA had 35 to 45% perfect identity with five thiolase genes from different eucaryotic sources over the entire length of the peptide.Inducible fatty acid-oxidizing systems are characteristic of procaryotes and eucaryotic microsomal systems. Growth of hepatocytes or yeasts in culture in medium containing longchain fatty acids causes a proliferation of peroxisomes and an induction of the enzymes required for long-chain fatty acid oxidation (29,31). A similar response is seen after analysis of tissues isolated from animals fed a high-fat diet (19,20). Current evidence is accumulating that the induction of fatty acid-oxidizing enzyme activities noted in peroxisomes is a result of increased mRNA levels (13). The bestcharacterized system of inducible fatty acid oxidation is that described for Escherichia coli, in which many structural genes and a regulatory gene, fadR, required for the betaoxidation of long-and medium-chain fatty acids have been characterized by classical and molecular genetic techniques (9,15,21,23,25,28). Parallels between the peroxisomal and E. coli systems can be drawn both at the level of regulation by induction and in terms of enzyme structure. In each case, fatty acyl-coenzyme A (CoA) thiolase activity is associated with a single polypeptide, while at least three additional enzyme activities required for the beta-oxidation of fatty acids are associated with a single multifunctional polypeptide.In E. coli, a multifunctional enzyme complex encoded within the fadBA operon exhib...