Bioenergetics of the archaea

In the late 1970s, on the basis of rRNA phylogeny, Archaea (archaebacteria) was identified as a distinct domain of life besides Bacteria (eubacteria) and Eucarya. Though forming a separate domain, archaea display an enormous diversity of lifestyles and metabolic capabilities. Many archaeal species a re adapted to extreme environments with respect to salinity, temperatures a round the boiling point of water, and/or extremely alkaline or acidic pH. T his has posed the challenge of studying the molecular and mechanistic bases on which these organisms can cope with such adverse conditions. This revie w considers our cumulative knowledge on archaeal mechanisms of primary ener gy conservation, in relationship to those of bacteria and eucarya. Although the universal principle of chemiosmotic energy conservation also holds for Archaea, distinct features have been discovered with respect to novel ion- transducing, membrane-residing protein complexes and the use of novel cofac tors in bioenergetics of methanogenesis. From aerobically respiring archaea , unusual electron-transporting supercomplexes could be isolated and functi onally resolved, and a proposal on the organization of archaeal electron tr ansport chains has been presented. The unique functions of archaeal rhodops ins as sensory systems and as proton or chloride pumps have been elucidated on the basis of recent structural information on the atomic scale. Whereas components of methanogenesis and of phototrophic energy transduction in ha lobacteria appear to be unique to archaea, respiratory complexes and the AT P synthase exhibit some chimeric features with respect to their evolutionar y origin. Nevertheless, archaeal ATP synthases are to be considered distinc t members of this family of secondary energy transducers. A major challenge to future investigations is the development of archaeal genetic transforma tion systems, in order to gain access to the regulation of bioenergetic sys tems and to overproducers of archaeal membrane proteins as a prerequisite f or their crystallization.