Genes for respiratory nitrite reduction (denitrification) of Pseudomonas stutzeri are clustered within 7 kbp. A 4.6-kbp Hind III-Kpn I fragment carrying nirS, the structural gene for cytochrome cd₁, was sequenced. An open reading frame immediately downstream of nirS codes for a 22.8-kDa protein with four heme c-binding motifs. Mutagenesis of this gene causes an apparent defect in electron donation to cytochrome cd1. Following this ORF are the structural genes for cytochrome c₅₅₂, cytochrome c₅₅₁, and ORF5 that codes for a 11.9-kDa monoheme protein. All cytochromes have a signal sequence for protein export.