SummaryRecombinant plasmids which carried portions of theEscherichia coli frdoperon were constructed and their expression examined byin vivocomplementation ofE. ColiMI 1443. This strain lacked a chromosomalfrdoperon and was unable to grow anaerobically on glycerol and fumarate. Introduction of all four fumarate reductase subunits intoE. coliMI1443 was essential for the restoration of growth. The FRD A, FRD B dimer (but neither subunit alone) was active in the benzyl viologen oxidase assay. Both FRD C and FRD D were required for membrane association of fumarate reductase and for the oxidation of reduced quinone analogues. Introduction intoE. coliMI1443 of thefrdABCandfrdDgenes on two separate plasmid vectors failed to restore anaerobic growth on glycerol and fumarate. Thus separation of the DNA coding for the FRD C and FRD D proteins affected the ability of fumarate reductase to assemble into a functional complex.