The nitric oxide regulated nor promoter of Paracoccus denitrificans

The promoter of the Paracoccus denitrificans nitric oxide reductase operon (norCBQDEF) has been characterized by primer extension and deletion analysi s. A major transcript that is detectable only in anaerobically grown cells initiates 43.5 bp downstream of the centre of a putative binding site for t he transcription factor NNR (nitrite and nitric oxide reductase regulator, which is known to regulate nor expression). A minor transcript initiates 12 1 bp upstream of the major transcript and is detectable in cells grown aero bically or anaerobically. Deletion derivatives of the nor promoter region w ore constructed and analysed in vivo using transcriptional fusions to the r eporter gene lacZ. Expression patterns from promoter deletions in a wild-ty pe strain and an nnr mutant confirmed that the minor transcript is NNR inde pendent, and makes a small contribution to nor expression under both aerobi c and anaerobic growth conditions. A deletion derivative truncated to withi n 7 bp of the putative NNR-binding site showed a near wild-type response to anaerobic growth, showing that no upstream DNA sequences are required for activation of the major promoter. Site-directed mutagenesis of the putative NNR-binding site confirmed that this is the major cis-acting sequence medi ating the anaerobic inducibility of nor expression.