SUMMARY 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 are adapted to extreme environments with respect to salinity, temperatures around the boiling point of water, and/or extremely alkaline or acidic pH. This has posed the challenge of studying the molecular and mechanistic bases on which these organisms can cope with such adverse conditions. This review considers our cumulative knowledge on archaeal mechanisms of primary energy 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 cofactors in bioenergetics of methanogenesis. From aerobically respiring archaea, unusual electron-transporting supercomplexes could be isolated and functionally resolved, and a proposal on the organization of archaeal electron transport chains has been presented. The unique functions of archaeal rhodopsins 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 halobacteria appear to be unique to archaea, respiratory complexes and the ATP synthase exhibit some chimeric features with respect to their evolutionary origin. Nevertheless, archaeal ATP synthases are to be considered distinct members of this family of secondary energy transducers. A major challenge to future investigations is the development of archaeal genetic transformation systems, in order to gain access to the regulation of bioenergetic systems and to overproducers of archaeal membrane proteins as a prerequisite for their crystallization.

A ferredoxin isolated from the archaeon Sulfolobus acidocaldarius strain DSM 639 has been shown to contain one [3Fe-4S]'+"' cluster with a reduction potential of -275 mV and one [4Fe-4Sl2+''+ cluster with a reduction potential of -529 mV at pH 6.4, in the temperature range 0-50°C. The monomer molecular mass was confirmed to be 10907.5 2 1.0 Da by electrospray mass spectrometry, as calculated from the published amino acid sequence [Minami, Y., Wakabayashi, S., Wada, K., Matsubara, H., Kerscher, L. & Oesterhelt, D. (1985) J. Biochem. (Tokyo) 97, 745-7511, while the holoprotein molecular mass was found to be 11 550 +-1.0 Da. The reduced [3Fe-4S]" cluster was also shown by direct electrochemistry and magnetic circular dichroic spectroscopy to undergo a one-proton uptake reaction as first Although this appears to be only the second reported case of protonation at or near the reduced [3Fe-4Sl0 cluster, its observation in S. acidoculdarius ferredoxin raises the question of the generality of this chemistry for 3Fe clusters. The similarity of the pK, to the estimated intracellular pH of S. acidocaldarius strongly suggests a physiological role for this process.Keywords: Fe-S clusters; Fe-S proteins; electrochemistry; EPR; Archaea.Suljblohus acidocaldarius (strain DSM 639) is an autotrophic archaeon which grows optimally at 7O0C-80"C and at pH 2-3 [I 1. It contains a substantial amount (up to 30 mg/100 g cells) of a ferredoxin (Sa Fd) which was shown to be the physiological electron acceptor from the 2-oxoacid :ferredoxin oxidoreductase [2]. The oxidative decarboxylation of 2-oxoacids in archaea has been proposed to proceed in two single electron-transfer steps, the evidence being that a stable intermediate, thought to be a (hydroxyethy1)thiamin diphosphate radical, has been observed by EPR, upon reacting pyruvate with the oxidised enzymes. The same process has been observed in S. acidocaldarius [2], Halobacterium hulobium [4] and Pyrococcus furiosus [5], each of which are archaea. By contrast, in eukaryotes and aerobic bacteria, the same reaction involves the concerted transfer of two electrons, using the well-characterised 2-oxoacid dehydrogenase multienzyme complex [6].The amino acid sequence of S. ucidocaldarius ferredoxin was determined by Minami et al. 131 logenetic relationship of archaea to bacteria. Two cysteine-rich regions were identified in a 103-amino-acid polypeptide :Cys45-Ile-Ala-Asp-Gly-Ser-Cys-Ile-Thr-Ala-Cys-Pro towards the N-terminal and Cys83-Ile-Phe-Cys-Met-Ala-Cys-Val-AsnVal-Cys-Pro towards the C-terminal. Since the ferredoxin was shown to contain 8-10 iron atoms and 6-8 inorganic sulfide ions/molecule [2], it was assumed to contain two iron-sulfur clusters, probably of the [4Fe-4Slz"l + type. Several unusual features of the sequence were reported: first, the presence of a methylated lysine residue at position 29, unique among ferredoxins; second, the presence of an aspartate residue at position 48, which replaces the second cysteine residue (Cys2) in the middle of the so-called ferredoxin motif o...

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Transitory H+ eiection from Sulfolobus acidocaldorius cells induced by oxygen pulses. and anaerobic H+ backflow were . _ _ __ investigated. Aerobic proton extrusion is inhibited by protonophores, by nigericin and by inhibitors of respiratory electron transport; it is stimulated by DCCD. In contrast, DCCD inhibits the rate of anaerobic H+ backflow. Aerobic proton extrusion is significantly enhanced by K+/valinomycin. Apparent H+/O ratios of 2.53 are measured. Proton extrusion generates large pH gradients (34 units) representing the major contribution to the total proton motive force across the plasma membrane of this thermoacidophilic archaebacterium.