The two membrane-bound respiratory nitrate reductases of Escherichia c
oil are encoded by distinct operons at two different loci, chlC and ch
lZ, on the chromosome. The chlZ locus includes a narK homologue, narU,
encoding a nitrite extrusion protein, and narZYWV encoding nitrate re
ductase Z. No apparent homologue to the narXL operon has been found. H
omology between narU and narK on the one hand and narZYWV and narGHJI
on the other hand is limited to the coding regions.