Regulation of Denitrification
Most denitrifying bacteria are facultative and preferentially generate ATP by aerobic respiration if oxygen is available. In those cases that have been studied, the genes encoding the enzymes of the denitrification pathway are not expressed in cultures growing aerobically. Studies with several model denitrifying bacteria have shown that the transcription of denitrification genes is regulated by proteins that respond to environmental signals (principally oxygen and nitric oxide). The regulatory proteins and mechanisms are diverse, but there are dominant roles for proteins from the FNR/CRP and sensor–regulator families. Signal-sensing mechanisms typically depend on protein-bound haem, non-haem iron and iron–sulphur clusters. This chapter reviews the current understanding of the regulation of denitrification genes in selected model organisms. The focus is on the roles and mechanisms of transcriptional regulators and the topologies of regulatory networks. Consideration of a range of organisms reveals a diversity of protein molecules and mechanisms, but also some overarching themes. The regulation of nitric oxide detoxification is also considered briefly, since there are interesting overlaps and contrasts with the regulation of respiratory nitric oxide reduction.