CELL BIOLOGY AND MOLECULAR-BASIS OF DENITRIFICATION

Denitrification is a distinct means of energy conservation making use of N oxides as terminal electron acceptors for cellular bioenergetics under anaerobic, microaerophilic, and occasionally aerobic conditions. The process is an essential branch of the global N cycle,reversing di nitrogen fixation, and is associated with chemolithotrophic, phototrop hic, diazoatrophic, or organotrophic metabolism but generally not with obligately anaerobic life. Discovered more than a century ago and bel ieved to be exclusively a bacterial trait, denitrification has now bee n found in halophilic and hyperthermophilic archaea and in the mitocho ndria fungi, raising evolutionarily intriguing vistas. Important advan ces in the biochemical characterization of denitrification and the und erlying genetics have been achieved with Pseudomonas stutzeri, Pseudom onas aeruginosa, Paracoccus denitrificans, Ralstonia eutropha, and Rho dobacter sphaeroides. Pseudomonads represent one of the largest assemb lies of the denitrifying bacteria within a single genus favoring their rise as model organisms. Around 50 genes are required within a single bacterium to encode the core structures of the denitrification appara tus. Much of the denitrification process of gram-negative bacteria has been found confined to the periplasm, whereas the topology and enzymo logy of the gram-positive bacteria are less well established. The acti vation and enzymatic transformation of N oxides is based on the redox chemistry of Fe, Cu, and Mo. Biochemical breakthroughs have included t he X-ray structures of the two types of respiratory nitrite reductases and the isolation of the novel enzymes nitric oxide reductase and nit rous oxide reductase, as well as their structural characterization by indirect spectroscopic means. This revealed unexpected relationships a mong denitrification enzymes and respiratory oxygen reductases. Denitr ification is intimately related to fundamental cellular processes that include primary and secondary transport, protein translocation, cytoc hrome c biogenesis, anaerobic gene regulation, metalloprotein assembly and the biosynthesis of the cofactors molybdopterin and henze D-1. An important class of regulators for the anaerobic expression of the den itrification apparatus are transcription factors of the greater FNR fa mily. Nitrate and nitre oxide, in addition to being respiratory substr ates, have been identified as signaling molecules for the induction of distinct N oxide-metabolizing enzymes.