Proton transport by halorhodopsin

In halorhodopsin from Natronobacterium pharaonis, a light-driven chloride pump, the chloride binding site also binds azide. When azide is bound at this location the retinal Schiff base transiently deprotonates after photoexcitation with light > 530 nm, like in the light-driven proton pump bacteriorhodopsin. As in the photocycle of bacteriorhodopsin, pyranine detects the release of protons to the bulk. The subsequent reprotonation of the Schiff base is also dependent on azide, but with different kinetics that suggest a shuttling of protons from the surface as described earlier for halorhodopsin from Halobacterium salinarium. This azide-dependent, bacteriorhodopsin-like photocycle results in active electrogenic proton transport in the cytoplasmic to extracellular direction, detected in cell envelope vesicle suspensions both with a potential-sensitive electrode and by measuring light-dependent pH change. We conclude that in halorhodopsin an azide bound to the extracellular side of the Schiff base, and another azide shuttling between the Schiff base and the cytoplasmic surface, fulfill the functions of Asp-85 and Asp-96, respectively, in bacteriorhodopsin. Thus, although halorhodopsin is normally a chloride ion pump, it evidently contains all structural requirements, except an internal proton acceptor and a donor, of a proton pump. This observation complements our earlier finding that when a chloride binding site was created in bacteriorhodopsin through replacement of Asp-85 with a threonine, that protein became a chloride ion pump.

Rhodamine 123 efflux transporter in Haloferax volcanii is induced when cultured under 'metabolic stress' by amino acids: the efflux system resembles that in a doxorubicin-resistant mutant

In this paper, we report that an archaebacterium, Haloferax volcanii, cultured in medium containing a large excess of amino acids showed very low levels of rhodamine 123 (RH123), which is a potent substrate for P-glycoprotein and the bacterial multidrug efflux transporter. This low level involved the active efflux of RH123 from the cells. The level of intracellular RH123 was increased and the efflux inhibited by the Ca2+-channel antagonist verapamil and also by various anti-cancer drugs. The efflux transporter was suggested to be ATP-driven. We have previously selected a mutant of H. volcanii with resistance to doxorubicin, by repeatedly culturing cells in 1.5 microM doxorubicin [Miyauchi, Komatsubara and Kamo (1992) Biochim. Biophys. Acta 1110, 144-150]. The acquisition of resistance to doxorubicin involves the active expulsion of lipophilic drugs such as RH123 and doxorubicin. It is notable that the drug spectrum and ATP-dependency of the amino acid-induced efflux transporter resemble those of the efflux transporter induced by doxorubicin.

Total reconstitution of active small ribosomal subunits of the extreme halophilic archaeon Haloferax mediterranei

The small ribosomal subunit of the halophilic archaeon Haloferax mediterranei has been reconstituted from its dissociated rRNA and protein components. Efficient reconstitution of particles, fully active in poly(U)-dependent polyphenylalanine synthesis, occurs after 2 h of incubation at 36 degrees C in the presence of 1.5 M of (NH4)2SO4, 100 mM of MgAc2, 20 mM Tris-HCl (pH 8.2) and 6 mM 2-mercaptoethanol. Important differences in the optimal ionic conditions for the reconstitution of the 30S and the 50S ribosomal subunits from Haloferax mediterranei have been found. K+ and NH4+ ions have differing abilities to promote the reconstitution of the particles. The assembly of 30S ribosomal subunits of H. mediterranei has a higher tolerance to ionic strength than the assembly of the 50S subunits and it is independent of the Mg2+ concentration present in the system.

Diversity of lactate metabolism in halophilic archaea

D-Lactate is readily used as a substrate for the growth of species of halophilic archaea belonging to the genera Haloferax and Haloarcula. L-Lactate was used by Haloferax species (Haloferax volcanii, Haloferax mediterranei) only when a substantial concentration of the D-isomer was also present in the medium. On the enzymatic level, considerable diversity was found in the lactate metabolism of the different representatives of the Halobacteriaceae. At least three types of lactate dehydrogenases were detected in halophilic archaea. A high level of activity of an NAD-linked enzyme was present constitutively in Haloarcula species, and a low level of activity was also detected in Haloferax mediterranei. NAD-independent lactate dehydrogenases, oxidizing L-lactate and D-lactate with 2,6-dichlorophenol-indophenol as electron acceptor, were detected in all nine species tested, but L-lactate dehydrogenase activity in Halobacterium species was very low, and Haloarcula species, which possess a high level of activity of NAD-linked lactate dehydrogenase, showed very low activities of both NAD-independent D- and L-lactate dehydrogenase. An inducible lactate racemase, displaying an unusually high pH optimum, was found in Haloferax volcanii. Lactate racemase activity was found constitutively in Haloarcula species, but no activity was detected in Halobacterium species and in Haloferax mediterranei.

Shape of the chromophore binding site in pharaonis phoborhodopsin from a study using retinal analogs

To investigate the shape of the chromophore binding site of pharaonis phoborhodopsin (ppR), ppR-opsin was incubated with five ring-modified retinal analogs: an acyclic retinal, phenylretinal, alpha-retinal, cyclohexylretinal and 5-isopropyl-alpha-retinal. The experimental results were compared with those obtained from bacteriorhodopsin-opsin (bR-opsin) and the same retinal analogs. It was suggested that ring chain conformation is important in affecting the spectral shoulder unique for the absorption spectrum of ppR. The rate of pigment formation depended greatly on the analogs used with the planar analogs showing rapid formation. Thus, we concluded that the space of the retinal binding site of ppR is restricted to the plane of the cyclohexenyl ring of the chromophore, whereas that of bR is less restricted.

Expression of a yeast intron-containing tRNA in the archaeon Haloferax volcanii

Expression of the yeast tRNAPro(UGG) gene in Haloferax volcanii resulted in the production of a single stable transcript that had not undergone intron processing or processing of 5' and 3' flanking sequences. Mutation of the exon-intron boundary region of this RNA to produce a precursor RNA with the preferred halobacterial consensus exon-intron boundary structure did not restore intron processing. Processing of 5' and 3' flanking sequences was restored when the acceptor stem U6-U67 pair was changed to A6-U67. The significance of these results in defining the recognition requirements of tRNA maturation enzymes in the halophilic domain Archaea is discussed.

Phototactic behaviour of the archaebacterial Natronobacterium pharaonis

Natronobacterium pharaonis can react tactically to photo- and chemostimuli. It moves by rotation of a flagellar bundle which is monopolarly inserted. Under sufficient oxygen supply the photophobic response of N. pharaonis has been measured. The resulting action spectrum matches the absorption spectrum of the purified retinylidene protein psR-II. Retical synthesis could be inhibited by nicotine. Cells grown in the presence of nicotine show a strongly reduced photoresponse, which could be restored by addition of retinal. These data identify psR-II as the receptor for negative phototaxis.

Stringency and relaxation among the halobacteria

Accumulation of stable RNA and production of guanosine polyphosphates (ppGpp and pppGpp) were studied during amino acid starvation in four species of halobacteria. In two of the four species, stable RNA was under stringent control, whereas one of the remaining two species was relaxed and the other gave an intermediate phenotype. The stringent reaction was reversed by anisomycin, an effect analogous to the chloroamphenicol-induced reversal of stringency in the eubacteria. During the stringent response, neither ppGpp nor pppGpp accumulation took place during starvation. In both growing and starved cells a very low basal level of the two polyphosphates appeared to be present. In the stringent species the intracellular concentration of GTP did not diminish but actually increased during the course of the stringent response. These data demonstrate that (i) wild-type halobacteria can have either the stringent or the relaxed phenotype (all wild-type eubacteria tested have been shown to be stringent); (ii) stringency in the halobacteria is dependent on the deaminoacylation of tRNA, as in the eubacteria; and (iii) in the halobacteria, ppGpp is not an effector of stringent control over stable-RNA synthesis.

Reduction of nitrosubstituted aromatic compounds by the halophilic anaerobic eubacteria Haloanaerobium praevalens and Sporohalobacter marismortui

The moderately halophilic, obligately anaerobic eubacteria Haloanaerobium praevalens DSM 2228 and Sporohalobacter marismortui ATCC 35420 are able to reduce a variety of nitrosubstituted aromatic compounds at a high rate to the corresponding amines. Compounds degraded included nitrobenzene, o-nitrophenol, m-nitrophenol, p-nitrophenol, nitroanilines, 2,4-dinitrophenol, and 2,4-dinitroaniline. Most of these compounds, when added at concentrations of 50 to 100 mg/liter, were completely transformed within 24 h, but at the highest concentrations growth rates were somewhat lowered. Growth of H. praevalens in the presence of 14C-labeled p-nitrophenol showed that the compound was not incorporated by the cells or degraded to acid-volatile compounds.

Anaerobic degradation of organic compounds at high salt concentrations

A number of obligately anaerobic fermentative bacteria are known to degrade a variety of organic substrates such as sugars, amino acids, and others, in the presence of high salt concentrations (up to 3-4 M) to products such as hydrogen, CO2, acetate and higher fatty acids, and ethanol. Our understanding of the fate of these products in hypersaline environments is still extremely limited. The occurrence of bacterial sulfate reduction is well established at salt concentrations of up to 24%; however, the bacteria involved have not yet been isolated in pure culture, and the range of electron donors used is unknown. Halophilic or halotolerant methanogenic bacteria using hydrogen/CO2 or acetate as energy source are notably absent; methanogenesis under hypersaline conditions is probably limited to such substrates as methanol and methylamines, which cannot be expected to be major products of anaerobic degradation of most organic compounds.

Survival strategies of microorganisms in extreme saline environments

Halophilic representatives are found in all main lines of evolutionary descendence of microbes: in archaebacteria, Gram-negative and Gram-positive eubacteria, and also in eucaryotes. In principle all halophilic microorganisms have to adapt their surface and membrane structures to their highly ionic environments. Concerning their intracellular compartment two different strategies have been developed: Inorganic ions are largely excluded in some microorganisms while such ions are actively accumulated in others. In particular the second group of organisms has to adapt the whole metabolic machinery to the highly ionic conditions of several molar salts, whereas in the first group only the outer surface of the cytoplasmic membrane and the extracytoplasmic structures are in contact with high concentrations of inorgainic ions. In this latter group, a variety of organic solutes is accumulated in response to increases of the salinity of the environment.

Polyamines in photosynthetic eubacteria and extreme-halophilic archaebacteria

Qualitative and quantitative determinations of polyamines have been done in 4 photosynthetic eubacteria and 6 extreme-halophilic archaebacteria. For comparison, 5 moderate-halophilic eubacteria were also analyzed to determine their polyamine contents. Not only putrescine and spermidine but also homospermidine were found in the photosynthetic eubacteria, especially in the N2-fixing species, Rhodospirillum and Chromatium. Norspermidine, norspermine, and spermine were not detected in the phototrophic eubacteria. No appreciable amount of any polyamine was found in extreme-halophilic archaebacteria, Halobacterium and Halococcus, while moderate-halophilic eubacteria contained quite high concentrations of putrescine and spermidine and cadaverine. When arginine was incubated with cell lysates of these two archaebacteria, appreciable amounts of agmatine were produced; neither putrescine nor cadaverine was formed in the presence of ornithine or lysine. No detectable amount of spermidine was produced by the lysates on incubation with putrescine.

[Absorption of 14C-dicarboxylic acids by bacteria of the family Halobacteriaceae]

Cultures of the family Halobacteriaceae belonging to the species Halobacterium halobium, H. cutirubrum, H. vallismortis and Halococcus morrhuae were shown to be capable of assimilating 14C-succinate. Halobacterium salinarium lacked this ability. The transport systems of C4-dicarboxylates differed in Halobacterium halobium 996 and H. vallismortis 1398, on the one hand, and Halococcus morrhuae 1235, on the other. The differences involve the kinetic parameters and stereospecificity of transport systems, the ability to take up different labelled C4-dicarboxylates, the pH-dependence of transport, and the action of CCCP, a protonophorous uncoupling agent. Halobacteria are capable of labelled succinate uptake at a lower NaCl content in the incubation medium than it is necessary for their growth. The optimal temperature for 14C-succinate uptake by halobacteria is higher than the optimal temperature of their growth. For all of the studied cultures, the transport system of dicarboxylate was shown to differ from that of E. coli common for C4-dicarboxylates and aspartate.

[Comparative characteristics of the transport systems of C4-dicarboxylic acids in cultures of the genera Halobacterium and Halococcus]

The transport systems of the extreme halophilic organisms, Halobacterium and Halococcus, differ to a considerable degree in the kinetic parameters of succinate and fumarate transport, the exchange between exocellular and endocellular labeled dicarboxylates, and the rate at which labeled compounds in the cell are incorporated into cellular metabolism.

Effects of NaCl on the uptake of alpha-[14C]aminoisobutyric acid by the halotolerant bacterium Halomonas elongata

The alpha-aminoisobutyric acid (AIB) transport system of the halotolerant bacterium, Halomonas elongata, was examined. Cells were grown in L-alanine defined medium with 0.05, 0.375, 1.37, 2.5, or 3.4 M NaCl. Each group of cells was resuspended in buffered salts with different NaCl concentrations (0.05, 0.375, 1.37, 2.5, and 3.4 M) and the uptake of alpha-[14C]AIB was measured. Optimum AIB uptake occurred in the 0.375 M NaCl solution for the lower salt grown cells and the 1.37 M NaCl solution for the higher salt grown cells. When cells were grown in the higher salt media and suspended in hypoosmotic solutions, appreciable AIB uptake occurred; but for cells grown in lower salt media and suspended in hyperosmotic solutions, the uptake was dramatically reduced. This effect was mainly attributed to cell plasmolysis which in turn resulted in some cell death. The AIB uptake was Na+ specific and this analogue was not metabolized after being transported into the cells. An amino acid competition study gave a pattern similar to that of a marine bacterium.

[Electron transport chain in a thermophilic methane-oxidizing culture of Methylococcus thermophilus]

The electron transport chain was studied in the obligate methane oxidizing culture of Methylococcus thermophilus during the oxidation of methanol (the source of carbon) which is an oxidized derivative of methane as well as during the oxidation of hydroxylamine which is an intermediate in the oxidation of ammonium (the source of nitrogen) by Mc. thermophilus cells. Cytochromes a, b and c are involved in electron transport. Cytochrome cco and cytochrome c554 have been isolated from the cell-free extract of Mc. thermophilus and purified. A scheme for electron transport operating in the oxidation of methanol and hydroxylamine is suggested on the basis of studying the characteristics of these cytochromes. Cytochrome a was shown to be a component of terminal oxidase. Cytochromes b are connected with membranes and also found in the composition of hydroxylamine oxidase. Cytochrome cco and, possibly, terminal oxidase (cytochromes a) are involved, in the oxidation of CH3OH by methanol dehydrogenase, in electron transport; cytochrome c554 as well as cytochrome b and c in the composition of hydroxylamine oxidase participate in electron transport in the oxidation of NH2OH by hydroxylamine oxidase. The characteristics of the electron transport system in Mc. thermophilus are discussed.