Immunocytochemical detection and location of the membrane-bound nitrite oxidoreductase in cells of Nitrobacter and Nitrospira

The intracellular location of the membrane-bound nitrite oxidoreductase of Nitrobacter hamburgensis X14 was determined by electron microscopic immunocytochemistry. Post-embedding labeling performed on ultrathin sections revealed that the enzyme was located at the cytoplasmic membrane and at the intracytoplasmic membranes. As shown by immunonegative staining of isolated membranes the labeling of the monoclonal antibodies named Hyb 153-2 and Hyb 153-3 was located at the cytoplasmic side of the membranes. Using Hyb 153-2, which recognize the α-subunit of nitrite oxidoreductase, the labeling was found at the surface of the particulate membranes. In contrast, Hyb 153-3, recognizing the β-subunit of nitrite oxidoreductase, bound preferably at the periphery of the membrane fragments. Besides Nitrobacter, Hyb 153-3 also recognized the nitrite-oxidizing system in two strains of Nitrospira. No reaction was found with Nitrospina and Nitrococcus.