Mutational analysis of the nucleotide sequence at the FNR-dependent nirB promoter in Escherichia coli.

During anaerobic growth of E. coli, the FNR protein activates transcription initiation at the nirB promoter. After chemical synthesis using deliberately contaminated nucleotides, we isolated a series of recombinant plasmids with single point mutations or one base pair deletions in the nirB promoter. The effects of these alterations on the anaerobic induction of promoter activity were measured. Mutations that abolish anaerobic induction identify the -10 hexamer sequence whilst changes that allow reduced induction suggest positions involved in FNR binding. Comparison of the nucleotide sequence of the nirB promoter with other promoters that are regulated by FNR show clear homologies, suggesting consensus sequences for FNR binding sites, and confirming that some of the point mutations described here do indeed act by weakening FNR binding.