Biochemical and genetic characterization of nirB mutants of Escherichia coli K 12 pleiotropically defective in nitrite and sulphite reduction.

Mutants of Escherichia coli K12 defective in the nirB gene lack NADH-dependent nitrite reductase activity and reduce nitrite slowly during anaerobic growth. With one exception these mutants require cysteine for growth. Cytochrome C552 synthesis and the assimilation of ammonia are unaffected by the nirB mutation. The defective gene is located between the crp and aroB genes at minute 73 on the E. coli chromosome. Mapping and reversion studies indicate the nirB is identical to the previously described cysG gene. It is suggested that the product of the cysG+ (nirB+)¿gene is an enzyme required for the synthesis of sirohaem, a prosthetic group of enzymes which catalyse the six-electron reduction of nitrite to ammonia and sulphite to sulphide.