Membrane cytochromes of Escherichia coli chl mutants

Abstract

The cytochromes present in the membranes of Escherichia coli cells having defects in the formate dehydrogenase-nitrate reductase system have been analyzed by spectroscopic, redox titration, and enzyme fractionation techniques. Four phenotypic classes differing in cytochrome composition were recognised. Class I is represented by strains with defects in the synthesis or insertion of molybdenum cofactor. Cytochromes of the formate dehydrogenase-nitrate reductase pathway are present. Class II strains map in the chlC-chlI region. The cytochrome associated with nitrate reductase (cytochrome b(nr)) is absent in these strains, whereas that associated with formate dehydrogenase (cytochrome b(fdh)) is the major cytochrome in the membranes. Class III strains lack both cytochromes b(fdh) and b(nr) but overproduce cytochrome d of the aerobic pathway even under anaerobic conditions in the presence of nitrate. Class III strains have defects in the regulation of cytochrome synthesis. An fdhA mutant produced cytochrome b(nr) but lacked cytochrome b(fdh). These results support the view that chlI (narI) is the structural gene for cytochrome b(nr) and that chlC (narG) and chlI (narI) are in the same operon, and they provide evidence of the complexity of the regulation of cytochrome synthesis.