TABLE 2.

Prokaryotic nitrite reduction

Characteristic	Assimilatory, NO2− assimilation	Dissimilatory
		NO2− respiration		NO2− reduction
		Nir	Nrf
Nitrite reductase	Assimilatory Nas	Respiratory Nir	Respiratory Nrf	Dissimilatory Nir
Location	Cytoplasm	Periplasm	Periplasm	Cytoplasm
Reaction catalyzed	NO2−⇒NH4+	NO2−⇒NO	NO2−⇒NH4+	NO2−⇒NH4+
Structural genes	nasBa/nirAb	nirS/nirK	nrfA	nirBD
Prosthetic groups	FADc, FeSd, siroheme	cytcd1e/Cu	cytc	FAD, FeS, siroheme
Nitrite transport	Yes	No	No	Yes
Function	Biosynthesis of N compounds	PMF (denitrification)	PMF (ammonification)	2H ⇓f and nitrite detoxification
Regulationg
O2	No	Yes	Yes	Yes
NH4+	Yes	No	No	No
NO3−/NO2−	Yes	Yes	Yes	Yes

^aFollowing the gene designation in K. oxytoca for the NADH-nitrite reductase. 

^bFollowing the gene designation in cyanobacteria for the ferredoxin-nitrite reductase. 

^cFAD is present in the NADH-nitrite reductases, but it is absent from the cyanobacterial assimilatory ferredoxin-dependent nitrite reductase. 

^dFeS, iron-sulfur centers. 

^ecytcd₁, cytochrome cd₁ complex. 

^f2H ⇓, dissipation of reducing power. 

^gSome differences in regulation in prokaryotic organisms have been reported.