An activity from Escherichia coli membranes responsible for the modification of nitrate reductase to its precursor form.

An enzymatic activity which modifies nitrate reductase has been identified in the cytoplasmic membrane of Escherichia coli. This activity changes subunit B to a form with a slightly greater electrophoretic mobility on sodium dodecyl sulfate-polyacrylamide gels (B'). The B' polypeptide produced by this modifying enzyme was compared to an apparently identical polypeptide identified in the precursor form of nitrate reductase which can be found in the cytoplasm of all strains and in the membrane of mutants defective in the insertion of nitrate reductase. These B' polypeptides were all identical with respect to mobility on gradient sodium dodecyl sulfate gels and peptides produced by limited digests using trypsin, papain, and Staphylococcus aureus V8 protease. When compared to subunit B, the proteolytic gel maps of B' polypeptides showed minor differences. From the identity of the modified B' with precursor B', the ability to convert B into B' in vitro and the in vivo nature of B' as a precursor of B, it was concluded that the modification of B to B' is a reversible process and is due to the removal of one or more small nonprotein molecules.