Anaerobic cytochrome b1 in Escherichia coli: association with and regulation of nitrate reductase.

Nitrate reductase solubilized from the membrane of Escherichia coli by alkaline heat treatment was purified to homogeneity and used to prepare specific antibody. Nitrate reductase, precipitated by this antibody from Triton extracts of the membrane, contained a third subunit, not present in the purified enzyme used to prepare the antibody. This third subunit was identified as the cytochrome b1 apoprotein. This cytochrome is bound to nitrate reductase from wild-type E. coli in a ratio of 2 mol of cytochrome per mol of enzyme complex. In mutants unable to synthesize heme, this cytochrome b1 apoprotein is not bound to nitrate reductase. In these same mutants, the enzyme is overproduced and accumulates in the cytoplasm. The absence of cytochrome also affects the stability of the membrane-bound form of the enzyme.