Some species also contain __________ which can further reduce nitrite into nitric oxide.

Some species also contain __________ which can further reduce nitrite into nitric oxide.

• A. Nitrite reductase genes (Nir)
• B. Nitrate reductase genes (Nar)
• C. Nitrite transporter genes
• D. Nitric oxide synthase genes

Answer: (A)

In denitrification, the conversion of nitrite (NO2–) to nitric oxide (NO) is carried out by nitrite reductase. Genes encoding this enzyme (nir genes) enable the reduction of nitrite to nitric oxide, a key step after nitrate has already been reduced to nitrite by nitrate reductase. Some bacteria possess Nir to complete this part of the pathway, with two common forms being NirS and NirK in different organisms. In contrast, nitrate reductase (Nar) handles the earlier step of reducing nitrate to nitrite, nitrite transporters merely move nitrite across membranes, and nitric oxide synthase makes NO from arginine rather than from nitrite. Therefore, nitrite reductase genes best fit the description of enabling the reduction of nitrite to nitric oxide.

In denitrification, the conversion of nitrite (NO2–) to nitric oxide (NO) is carried out by nitrite reductase. Genes encoding this enzyme (nir genes) enable the reduction of nitrite to nitric oxide, a key step after nitrate has already been reduced to nitrite by nitrate reductase. Some bacteria possess Nir to complete this part of the pathway, with two common forms being NirS and NirK in different organisms. In contrast, nitrate reductase (Nar) handles the earlier step of reducing nitrate to nitrite, nitrite transporters merely move nitrite across membranes, and nitric oxide synthase makes NO from arginine rather than from nitrite. Therefore, nitrite reductase genes best fit the description of enabling the reduction of nitrite to nitric oxide.