The electron transport chain of Rhizobium trifolii

Identification of a novel gene for biosynthesis of a bacteroid-specific electron carrier menaquinone

Ubiquinone (UQ) has been considered as an electron mediator in electron transfer that generates ATP in Rhizobium under both free-living and symbiosis conditions. When mutated, the dmtH gene has a symbiotic phenotype of forming ineffective nodules on Astragalus sinicus. The gene was isolated from a Mesorhizobium huakuii 7653R transposon-inserted mutant library. The DNA sequence and conserved protein domain analyses revealed that dmtH encodes demethylmenaquinone (DMK) methyltransferase, which catalyzes the terminal step of menaquinone (MK) biosynthesis. Comparative analysis indicated that dmtH homologs were present in only a few Rhizobia. Real-time quantitative PCR showed dmtH is a bacteroid-specific gene. The highest expression was seen at 25 days after inoculation of strain 7653R. Gene disruption and complementation tests demonstrated that the dmtH gene was essential for bacteroid development and symbiotic nitrogen fixation ability. MK and UQ were extracted from the wild type strain 7653R and mutant strain HK116. MK-7 was accumulated under microaerobic condition and UQ-10 was accumulated under aerobic condition in M. huakuii 7653R. The predicted function of DmtH protein was confirmed by the measurement of methyltransferase activity in vitro. These results revealed that MK-7 was used as an electron carrier instead of UQ in M. huakuii 7653R bacteroids.

Genes involved in the formation and assembly of rhizobial cytochromes and their role in symbiotic nitrogen fixation

Rhizobia fix nitrogen in a symbiotic association with leguminous plants and this occurs in nodules. A low-oxygen environment is needed for nitrogen fixation, which paradoxically has a requirement for rapid respiration to produce ATP. These conflicting demands are met by control of oxygen flux and production of leghaemoglobin (an oxygen carrier) by the plant, coupled with the expression of a high-affinity oxidase by the nodule bacteria (bacteroids). Many of the bacterial genes encoding cytochrome synthesis and assembly have been identified in a variety of rhizobial strains. Nitrogen-fixing bacteroids use a cytochrome cbb3-type oxidase encoded by the fixNOQP operon; electron transfer to this high-affinity oxidase is via the cytochrome bc1 complex. During free-living growth, electron transport from the cytochrome bc1 complex to cytochrome aa3 occurs via a transmembrane cytochrome c (CycM). In some rhizobia (such as Bradyrhizobium japonicum) there is a second cytochrome oxidase that also requires electron transport via the cytochrome bc1 complex. In parallel with these cytochrome c oxidases there are quinol oxidases that are expressed during free-living growth. A cytochrome bb3 quinol oxidase is thought to be present in B. japonicum; in Rhizobium leguminosarum, Rhizobium etli and Azorhizobium caulinodans cytochrome d-type oxidases have been identified. Spectroscopic data suggest the presence of a cytochrome o-type oxidase in several rhizobia, although the absence of haem O in B. japonicum may indicate that the absorption attributed to cytochrome o could be due to a high-spin cytochrome b in a cytochrome bb3-type oxidase. In some rhizobia, mutation of genes involved in cytochrome c assembly does not strongly affect growth, presumably because the bacteria utilize the cytochrome c-independent quinol oxidases. In this review, we outline the work on various rhizobial mutants affected in different components of the electron transport pathways, and the effects of these mutations on symbiotic nitrogen fixation and free-living growth.

Rhizobium phaseoli cytochrome c-deficient mutant induces empty nodules on Phaseolus vulgaris L

A Rhizobium phaseoli cytochrome mutant, unable to oxidize N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD), was isolated after Mu-dl (Kan lac) mutagenesis of the wild-type strain CE-3. Mutant strain CFN4202 had sixfold less haem-c but similar levels of b type, o and aa3 cytochromes than the wild-type strain. CFN4202 strain also showed reduced NADH- and TMPD-oxidase activity than the wild-type strain. Succinate-oxidase activities were very similar. Western blot experiments, using antiserum against bovine c1 and c cytochromes, revealed that both proteins were present in CFN4202 membranes, suggesting a defect of haem binding to cytochrome c. Nodules formed by this strain in Phaseolus vulgaris did not contain bacteroids. These data suggest that the cytochrome c-aa3 chain or some other respiratory chain, containing c-type cytochromes in R. phaseoli, is essential for bacterial division during the early steps of the symbiotic interaction with the legume-host.

Branched respiratory chain in aerobically grown Staphylococcus aureus--oxidation of ethanol by cells and protoplasts

Addition of ethanol and some other primary alcohols, except methanol, to cells and protoplasts (but not membrane particles) considerably stimulated the rate of oxygen consumption. This additional respiration was strongly inhibited by 0.1 mM KCN. The cyanide inhibition curve of endogenous substrate oxidation was slightly biphasic while in the presence of ethanol it became clearly biphasic having Ki values of approx. 0.1 and 0.5 mM. Based on the steady-state cytochrome spectra in the presence of 0.1 mM KCN, we attributed the lower Ki to cytochrome a602. Proteolysis of protoplasts external membrane proteins did not change the rate of endogeneous substrate oxidation but prevented the inhibition of this respiration by low concentrations of KCN and stimulation of oxygen consumption by ethanol. The activity of NAD(+)-dependent ethanol dehydrogenase in the cytoplasm was found to be 520 nmol NADH- x min-1 x mg-1 protein. Proteolysis of external membrane proteins apparently inhibits the operation of the cytochrome a602-containing electron transport branch inducing the suppression of electron flow from NADH to oxygen.

Isolation of Rhizobium phaseoli Tn5-induced mutants with altered expression of cytochrome terminal oxidases o and aa3

Two Rhizobium phaseoli mutants affected in cytochrome expression were obtained by Tn5-mob mutagenesis of the wild-type strain (CE3). Mutant strain CFN031 expressed sevenfold less cytochrome o in culture, expressed cytochrome aa3 under microaerophilic culture conditions, in contrast to strain CE3, and was affected in its vegetative growth properties and proliferation inside plant host cells. Mutant CFN037 expressed cytochrome aa3 under microaerophilic culture conditions, while bacteroid development and nitrogen fixation occurred earlier than in strain CE3.

The distribution of electron flow in the branched respiratory chain of Micrococcus luteus

Endogenous coupled respiration of Micrococcus luteus protoplasts showed a relatively high resistance to low concentrations of KCN, 2-nonyl-4-hydroxyquinoline N-oxide (NQNO) and dicyclohexylcarbodi-imide (DCCD) when the inhibitors were applied individually. In the presence of both KCN and NQNO (or DCCD), O2 uptake was strongly inhibited. The proteolysis of external membrane proteins of protoplasts also induced the high sensitivity of endogenous coupled respiration to low KCN. The effects of NQNO, DCCD and proteolysis were explained by the inhibition of an alternative respiratory system when reducing equivalents passed preferentially down the KCN-sensitive cytochrome oxidase. Uncoupling of the cell membrane increased the electron flow via the cytochrome oxidase-containing respiratory branch. It is suggested that the energy state of cells could control the electron-flow distribution between two branches, and quinones of different levels of reduction could be involved in the mechanism of respiratory branching.

Isolation of a Rhizobium phaseoli cytochrome mutant with enhanced respiration and symbiotic nitrogen fixation

Cultured cells of a Rhizobium phaseoli wild-type strain (CE2) possess b-type and c-type cytochromes and two terminal oxidases: cytochromes o and aa3. Cytochrome aa3 was partially expressed when CE2 cells were grown on minimal medium, during symbiosis, and in well-aerated liquid cultures in a complex medium (PY2). Two cytochrome mutants of R. phaseoli were obtained and characterized. A Tn5-mob-induced mutant, CFN4201, expressed diminished amounts of b-type and c-type cytochromes, showed an enhanced expression of cytochrome oxidases, and had reduced levels of N,N,N',N'-tetramethyl-p-phenylenediamine, succinate, and NADH oxidase activities. Nodules formed by this strain had no N2 fixation activity. The other mutant, CFN4205, which was isolated by nitrosoguanidine mutagenesis, had reduced levels of cytochrome o and higher succinate oxidase activity but similar NADH and N,N,N',N'-tetramethyl-p-phenylenediamine oxidase activities when compared with the wild-type strain. Strain CFN4205 expressed a fourfold-higher cytochrome aa3 content when cultured on minimal and complex media and had twofold-higher cytochrome aa3 levels during symbiosis when compared with the wild-type strain. Nodules formed by strain CFN4205 fixed 33% more N2 than did nodules formed by the wild-type strain, as judged by the total nitrogen content found in plants nodulated by these strains. Finally, low-temperature photodissociation spectra of whole cells from strains CE2 and CFN4205 reveal cytochromes o and aa3. Both cytochromes react with O2 at -180 degrees C to give a light-insensitive compound. These experiments identify cytochromes o and aa3 as functional terminal oxidases in R. phaseoli.

Hydrogen oxidation and nitrogen fixation in rhizobia, with special attention focused on strain ORS 571

In this survey we describe the influence of hydrogen oxidation on the physiology of Rhizobium ORS 571. The presence of hydrogen is required for the synthesis of hydrogenase. Carbon substrates do not repress the synthesis of hydrogenase. The respiratory system contains cytrochromes of the b- and c-type. Cytochrome alpha 600 is present after growth at high oxygen tensions. The nature of the terminal oxidases functioning at low oxygen tensions has not been established yet----H+/O values with endogenous substrates are between 6 and 7. The results show the presence of two phosphorylation sites: site 1 (ATP/2e = 1.0) and site 2(ATP/2e = 1.33). By measuring molar growth yields it has been demonstrated that carbon-limited, nitrogen-fixing cultures obtain additional ATP from hydrogen oxidation, and that site 2 of oxidative phosphorylation is passed during hydrogen oxidation. A method is described to calculate ATP/N2 values (the total amount of ATP used by nitrogenase during the fixation of 1 mol N2) and H2/N2 ratios (mol hydrogen formed per mol N2 fixed) in aerobic organisms. For Rhizobium ORS 571 the ATP/N2 value is about 40 and the H2/N2 ratio is between 5 and 7.5. Cells obtained from oxygen-limited nitrogen-fixing cultures contain 30-40% poly-beta-hydroxybutyrate, which explains the high molar growth yields found. Hydrogen has not been detected in the effluent gas of these cultures, which may point to reoxidation of the hydrogen formed at nitrogen fixation. Calculations show that the effect of hydrogen reoxidation on the efficiency of nitrogen fixation (g N fixed X mol-1 substrate converted) is not very large and that the actual H2/N2 ratio is of much more importance. After addition of hydrogen to succinate-limited, ammonia-assimilating cultures, an initial increase of the Ysuccinate value (g dry wt X mol-1 succinate) is followed by a gradual decrease. This is accompanied by a large decrease of the YO2 value, and an increased permeability of the cytoplasmic membrane to protons. The results may be explained by a transition of the culture from an energy-limited state to a carbon-limited state.