A Fumarate Reductase in Escherichia coli Distinct from Succinate Dehydrogenase

We found that in another facultative anaerobe, Escherichia coli,’ the levels of these two activities also varied in a nonparallel manner when aerobically and anaerobically grown cells were compared. To determine unequivocally whether two distinct enzymes were produced for these reactions, we sought to dissociate the enzymes by eliminating one through mutation. Since these activities are particle-bound in E. coli (lo), this approach circumvents the difficulties and possible artifacts of solubilizing for fractionation. It also permits one to determine definitively the physiological role of an enzyme, since one can observe the effect in viva that results from its loss. The desired mutant was obtained, making possible the separation and characterization of two succinate-oxidizing and fumarate-reducing enzymes in E. coli.