NITRATE REDUCTASES IN ESCHERICHIA-COLI

Escherichia coli expresses two different membrane-bound respiratory ni trate reductases, nitrate reductase A (NRA) and nitrate reductase Z (N RZ). In this review, we compare the genetic control, biochemical prope rties and regulation of these two closely related enzyme systems. The two enzymes are encoded by distinct operons located within two differe nt loci on the E. coli chromosome. The narGHJI operon, encoding nitrat e reductaseA, is located in the chlC locus at 27 minutes, along with s everal functionally related genes: narK, encoding a nitrate/nitrite an tiporter, and the narXL operon, encoding a nitrate-activated, two comp onent regulatory system. The narZYWV operon, encoding nitrate reductas e Z, is located in the chlZ locus located at 32.5 minutes, a region wh ich includes a narK homologue, narU, but no apparent homologue to the narXL operon. The two membrane-bound enzymes have similar structures a nd biochemical properties and are capable of reducing nitrate using no rmal physiological substrates. The homology of the amino acid sequence s of the peptides encoded by the two operons is extremely high but the intergenic regions share no related sequences. The expression of both the narGHJI operon and the narK gene are positively regulated by two transacting factors Fnr and NarL-Phosphate, activated respectively by anaerobiosis and nitrate, while the narZYWV operon and the narU gene a re constitutively expressed. Nitrate reductase A, which accounts for 9 8% of the nitrate reductase activity when fully induced, is clearly th e major respiratory nitrate reductase in E. coli while the physiologic al role of the constitutively expressed nitrate reductase Z remains to be defined.