The Functions and Components of the Anaerobic Respiratory Electron Transport Systems in Rhodobacter Capsulatus
The traditional view of Rhodobacter capsulatus has been of an organism that could grow either phototrophically under anaerobic conditions or aerobically in the absence of illumination. Recognition of anaerobic respiration as a characteristic of R. capsulatus is relatively recent (Ferguson et al., 1987). Nitrate, dimethylsulphoxide (DMSO), trimethylamine-N-oxide (TMAO) and nitrous oxide have been identified as anaerobic electron acceptors, but not all strains can use each of these oxidants. In non-phototrophic bacteria the function of enzymes that allow electron transport to terminate with the reduction of an anaerobic electron acceptor is to permit non-fermentative growth in the absence of oxygen. In some instances this function also applies to the anaerobic respiratory electron transport pathways in R. capsulatus, but, as will be discussed in this paper, phototrophic growth can be facilitated by the possession of the capacity to reduce electron acceptors under anaerobic conditions. The presence in R. capsulatus of certain anaerobic electron transport pathways also provides opportunities to study these pathways because R. capsulatus is much better characterised in terms of its genetics and complement of electron transport proteins than many of the organisms that have been longer recognised to perform anaerobic respiration. R. capsulatus should not be thought of as distinct amongst phototrophs in its possession of anaerobic respiratory pathways; a strain of Rhodobacter sphaeroides catalyses the complete set of denitrification reactions from nitrate to nitrogen gas (Urata and Satoh, 1985; Ito et al., 1989) whilst several other genera of photosynthetic bacteria have been shown to reduce nitrous oxide (McEwan et al., 1985a). Thus it is probable that much of what is discussed here for R. capsulatus will also be applicable to other related organisms.
KeywordsNitrous Oxide Nitrate Reductase Anaerobic Respiration Cyclic Electron Transport Paracoccus Denitrificans
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