Localization of the cytochrome cd₁ and copper nitrite reductases in denitrifying bacteria

The locations of cytochrome cd₁ nitrite reductases in Pseudomonas aeruginosa and Pseudomonas fluorescens and copper nitrite reductases in Achromobacter cycloclastes and Achromobacter xylosoxidans were identified. Immunogold labeling with colloidal-gold probes showed that the nitrite reductases were synthesized exclusively in anaerobically grown (denitrifying) cells. Little immunogold label occurred in the cytoplasm of these four strains; most was found in the periplasmic space or was associated with cell membranes. Immunogold labeling of thin sections was superior to fractionation by osmotic shock for locating nitrite reductases. The results support models of denitrification energetics that require a periplasmic, not a cytoplasmic, location for nitrite reductases.