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The genes that encode the bc-type nitric-oxide reductase from Paracoccus denitrificans have been identified. They are part of a cluster of six genes (norCBQDEF) and arc found near the gene cluster that encodes the cd1-type nitrite reductase, which was identified earlier [de Boer, A. P. N., Reijnders. W. N. M., Kuenen. J. G., Stouthamer, A. H. & van Spanning, R. J. M. (1994) Isolation, sequencing and mutational analysis of a gene cluster involved in nitrite reduction in Paracoccus denitrificans, Antonio Leon wenhoek 66. 111-127]. norC and norB encode the cytochrome-c-containing subunit II and cytochromeb-containing subunit I of nitric-oxide reductase (NO reductase), respectively, norQ encodes a protein with an ATP-binding motif and has high similarity to NirQ from Pseudomonas stutzeri and Pseudomonas aerurginosa and CbbQ from Pseudomonas hydrogenothermophila. norE encodes a protein with five putafive transmembrane α-helices and has similarity to CoxIII, the third subunit of the αα3-type cytochromec oxidases. norF encodes a small protein with two putative transmembrane α-helices. Mutagenesis of norC, norB norQ and nord resulted in cells unable to grow anaerobically. Nitrite reductase and NO reductase (with succinate or ascorbate as substrates) and nitrous oxide reductase (with succinate as sub strate) activities were not detected in these mutant strains. Nitrite extrusion was detected in the medium. indicating that nitrate reductase was active. The norQ and norD mutant strains retained about 16% and 23% of the wild-type level of NorC, respectively. The norE and norF mutant strains had specific growth faits and NorC contents similar to those of the wild-type strain but had reduced NOR and NIR activities. indicating tirol their gene products arc involved in regulation of enzyme activity. Mutant strains containing the norCBQDEF region on the broad-host-range vector pEG400 were able to grow anaerobically, althongh at a lower specific growth rate and with lower NOR activity compared with the wild-type strain. [ABSTRACT FROM AUTHOR]