ABSTRACT
Introduction ................................................................................................................................ 162 Phylogeny of Denitrifying Microorganisms .............................................................................. 162 Analysis of Genes Involved in Denitrifi cation of Thermophiles ............................................... 163 Variation of Denitrifi cation Respiratory Chains ........................................................................ 166 Biochemical Properties of Purifi ed Denitrifi cation Pathway Enzymes ..................................... 167
Nitrate Reductase ............................................................................................................ 167 Nitrite Reductase ............................................................................................................ 169 Nitric Oxide Reductase ................................................................................................... 170 Nitrous Oxide Reductase ................................................................................................ 170
Concluding Remarks .................................................................................................................. 171 Acknowledgment ....................................................................................................................... 171 References .................................................................................................................................. 171
Denitrifi cation is an important part of the global nitrogen cycle, however, the extent that extremophilic bacteria and archaea contribute to this cycle is unclear as only few isolates are obtained in pure culture and even fewer genomes of these have been sequenced. This review focuses on the denitrifi cation pathway of thermophilic bacteria and archaea. While thermophilic and mesophilic denitrifi ers share the same type of pathway enzymes important differences exist with respect to the localization and cofactors of the denitrifi cation pathway enzymes. The most signifi cant difference is the exterior orientation of the archaeal nitrate reductase (Nar) that catalyzes the fi rst step in the denitrifi cation pathway. As a consequence of this orientation, archaeal Nars do not participate directly in the generation of the proton motive force. In the archaea, all denitrifi cation pathway enzymes are membrane-associated. This is similar to the pathway enzymes from a gram-positive Bacillus sp., only one of which has been studied biochemically in more detail. This review will provide insight into the details of denitrifi cation pathway genes and enzymes of selected thermophiles that have been characterized to date.
