An open reading frame, designated nirQ, was identified upstream of nirS, the structural gene for the respiratory nitrite reductase of Pseudomonas stutzeri ZoBell. Its derived gene product (275 amino acids, Mr = 30,544) shows similarity to the NtrC protein family of transcriptional activators. Deletion-replacement mutagenesis of the nirQ gene resulted in the simultaneous loss of nitrite reduction and NO reduction in vivo. However, both reductases were still synthesized, with only nitrite reductase being active in vitro. NO reductase was overproduced by a factor of about 2. Our results indicate that the systems for nitrite reduction and NO reduction are functionally coupled.