Presence in the 'silent' terminus region of the Escherichia coli K12 chromosome of cryptic gene(s) encoding a new nitrate reductase.

A cosmid complementing narG mutants defective in nitrate reductase activity was isolated from a genomic library of Escherichia coli. The restriction map of the insert differed from that of the narGHI operon. The new enzyme, termed NarZ, required molybdenum for activity. The expression of narZ was not affected by the factors controlling narGHI. Insertion mutations indicated that the narZ locus covered about 8 kb of DNA; narZ is located at 32.5 U on the chromosome, in the cotransduction gap near the replication terminus. Southern blot experiments under stringent conditions using narGHI or narZ DNA as probes revealed a large extent of homology, with a small area of very high homology. We propose that narZ and narGHI have descended from a common ancestor by gene duplication.