Lack of redox control of the anaerobically-induced nirB+ gene of Escherichia coli K-12.

Operon fusion strains of Escherichia coli K-12 have been used to demonstrate that transcription of the structural gene for NADH-dependent nitrite reductase is regulated by oxygen repression and induction by its substrate, nitrite. This two-stage regulation of nirB is totally dependent upon a functional Fnr protein. Unlike the Fnr-dependent fumarate reductase operon, nirB transcription is not repressed by nitrate. These results suggest that the Fnr protein is simply a positive control protein essential for the derepression of some, but not all, anaerobically-induced operons rather than a more general redox-sensitive regulator, as suggested by the redox control hypothesis for the regulation of gene expression in facultatively anaerobic bacteria.