Nitrous oxide production and consumption: regulation of gene expression by gas-sensitive transcription factors

Several biochemical mechanisms contribute to the biological generation of nitrous oxide (N 2 O). N 2 O generating enzymes include the respiratory nitric oxide (NO) reductase, an enzyme from the flavo-diiron family, and flavohaemoglobin. On the other hand, there is only one enzyme that is known to use N 2 O as a substrate, which is the respiratory N 2 O reductase typically found in bacteria capable of denitrification (the respiratory reduction of nitrate and nitrite to dinitrogen). This article will briefly review the properties of the enzymes that make and consume N 2 O, together with the accessory proteins that have roles in the assembly and maturation of those enzymes. The expression of the genes encoding the enzymes that produce and consume N 2 O is regulated by environmental signals (typically oxygen and NO) acting through regulatory proteins, which, either directly or indirectly, control the frequency of transcription initiation. The roles and mechanisms of these proteins, and the structures of the regulatory networks in which they participate will also be reviewed.