Transcription initiation at the Escherichia coli nirB promoter is induced by anaerobic growth and further increased by the presence of nitrite or nitrate in the growth medium. Expression from this promoter is totally dependent on the transcription factor, FNR, which binds between positions -52 and -30 upstream of the transcription startsite. The 20 base pairs from position -79 to -60 contain an inverted repeat of two 10-base sequence elements that are related to sequences at the NarL-binding site at the E. coli narG promoter. Comparison of these, and sequence elements at other promoters regulated by NarL, suggests a consensus NarL-binding sequence. Mutations in the putative NarL-binding site at the nirB promoter decrease FNR-dependent anaerobic induction, suggesting that NarL acts as a helper to FNR during transcription activation. These mutations also suppress induction by nitrite: single mutations at symmetry-related positions have similar effects, whilst double mutations have more severe effects, probably because two NarL subunits bind to the inverted repeat. Disruption of narL decreases nitrite induction of the nirB promoter whilst not suppressing induction by nitrate, suggesting that there may be a second nitrate-responsive factor. Nitrate induction was, however, suppressed by double mutations at symmetry-related positions in the NarL-binding site, suggesting that this putative second factor may bind to sequences similar to those recognized by NarL.