SummaryThe regulatory region of two divergently oriented transcriptional units involved in the formation of the gas‐evolving hydrogenase (isoenzyme 3) ofEscherichia coliwas investigated. DNA sequence analysis revealed the existence of a 210bp non‐coding region containing two sequences showing homology to ‐24/‐12 NtrA‐dependent promoters. These sequences were arranged in a divergent orientation entirety consistent with their being involved in transcribing the divergent operons. Through S1 protection experiments it could be shown that transcription of both promoters was NtrA‐dependent and that it was regulated in an identical manner: oxygen repressed expression, as did anaerobic growth in the presence of nitrate; transcription was induced in cells grown anaerobically in the absence of exogenous electron acceptors and formate was found to be obligately required for this anaerobic induction. Lying at an approximately equal distance between both promoters was a short stretch of DNA which showed similarity to the sequence previously identified (Birkmann and Böck, 1989a) as being necessary for formate induction of