Identity of succinate dehydrogenase in chemotrophically and phototrophically grown Rhodospirillum rubrum

Overlapping role of the outer membrane cytochromes of Shewanella oneidensis MR-1 in the reduction of manganese(IV) oxide

AIM

To determine if the outer membrane (OM) cytochromes OmcA and OmcB of the metal-reducing bacterium Shewanella oneidensis MR-1 have distinct or overlapping roles in the reduction of insoluble manganese(IV) oxide.

METHODS AND RESULTS

The gene replacement mutant (OMCA1) which lacks OmcA was partially deficient in Mn(IV) reduction. Complementation of OMCA1 with a vector (pVK21) that contains omcB but not omcA restored Mn(IV) reduction to levels that were even greater than those of wild-type. Examination of the OM of OMCA1/pVK21 revealed greater than wild-type levels of OmcB protein and specific haem content.

CONCLUSIONS

Overexpression of OmcB can compensate for the absence of OmcA in the reduction of insoluble Mn(IV) oxides. Therefore, there is at least a partial overlap in the roles of these OM cytochromes in the reduction of insoluble Mn(IV) oxide.

SIGNIFICANCE

The overlapping roles of these two cytochromes has important implications for understanding the mechanism by which MR-1 reduces insoluble metal oxides. There is no obligatory sequential electron transfer from one cytochrome to the other. They could both potentially serve as terminal reductases for extracellular electron acceptors.

Chromium(VI) reductase activity is associated with the cytoplasmic membrane of anaerobically grown Shewanella putrefaciens MR-1

Shewanella putrefaciens MR-1 can reduce a diverse array of compounds under anaerobic conditions, including manganese and iron oxides, fumarate, nitrate, and many other compounds. These reductive processes are apparently linked to a complex electron transport system. Chromium (Cr) is a toxic and mutagenic metal and bacteria could potentially be utilized to immobilize Cr by reducing the soluble and bioavailable state, Cr(VI), to the insoluble and less bioavailable state, Cr(III). Formate-dependent Cr(VI) reductase activity was detected in anaerobically grown cells of S. putrefaciens MR-1, with highest specific activity in the cytoplasmic membrane. Both formate and NADH served as electron donors for Cr(VI) reductase, whereas L-lactate or NADPH did not support any activity. The addition of 10 micromol l(-1) FMN markedly stimulated formate-dependent Cr(VI) reductase, and the activity was almost completely inhibited by diphenyliodonium chloride, an inhibitor of flavoproteins. Cr(VI) reductase activity was also inhibited by p-chloromercuriphenylsulphonate, azide, 2-heptyl-4-hydroxyquinolone-N-oxide, and antimycin A, suggesting involvement of a multi-component electron transport chain which could include cytochromes and quinones. Cr(V) was detected by electron paramagnetic resonance (EPR) spectroscopy, suggesting a one-electron reduction as the first step.

Isolation and sequence of omcA, a gene encoding a decaheme outer membrane cytochrome c of Shewanella putrefaciens MR-1, and detection of omcA homologs in other strains of S. putrefaciens

The sequence of the omcA gene, which encodes a decaheme cytochrome c that is localized to the outer membrane (OM) of Shewanella putrefaciens MR-1, was determined. The 2202 bp nucleotide sequence of omcA encodes for 734 amino acids with a predicted molecular protein mass of 78.6 kDa. Comparison with the amino-terminal sequence of the mature protein suggests the presence of a hydrophobic leader sequence which is cleaved during translocation of the protein to the OM. This leader sequence has a lipoprotein consensus sequence for signal peptidase II at the cleavage site. The predicted mature protein is comprised of 708 amino acids with a predicted molecular mass of 75.8 kDa, but the addition of ten covalently attached heme c groups and covalent lipid modification to the amino-terminal cysteine increases the predicted mass to 82.7 kDa. This is consistent with its apparent mass of 83 kDa in SDS-PAGE gels. The predicted amino acid sequence for the OmcA protein shows no significant homology to known proteins. A RNA of approx. 2300 bases that hybridizes to the omcA gene was detected in anaerobically grown MR-1 cells. The size of this transcript is similar to the coding region of the omcA gene, suggesting that it is not part of a multicistronic operon. Similar to MR-1, four other strains of S. putrefaciens were all found to localize a majority of their membrane-bound cytochromes to the OM when grown under anaerobic conditions, and all contained an OM cytochrome of similar size to OmcA. In two of these strains, MR-4 and MR-8, a homolog of omcA was identified by RT-PCR and Southern blotting using primers and probes specific for omcA of MR-1. Western blot analysis using a polyclonal antibody to OmcA was similarly positive in strains MR-4 and MR-8. Partial nucleotide sequence analysis of these homologs demonstrated 74-77% predicted amino acid homology with OmcA of MR-1. In contrast, strains MR-30 and MR-42 tested negative for omcA homologs by Southern and Northern blots, RT-PCR, and Western blots.

Outer membrane cytochromes of Shewanella putrefaciens MR-1: spectral analysis, and purification of the 83-kDa c-type cytochrome

The metal-reducing bacterium Shewanella putrefaciens MR-1 is known to localize a majority of its membrane-bound cytochromes to its outer membrane when grown under anaerobic conditions. In this study, pyridine hemochrome spectra confirmed that these outer membrane cytochromes are c-type, and electrophoretic data demonstrated the presence of four distinct outer membrane cytochromes, with apparent molecular masses of 150, 83, 65, and 53 kDa. Fourth-order derivative analysis of 77 K spectra of the outer membrane revealed four spectrally distinct c-type hemes, with peaks at 545.4, 548.0, 550.6, and 552.6 nm. Outer membrane cytochromes in the reduced state were rapidly re-oxidized by oxidized iron and manganese, which have previously been shown to serve as electron acceptors for anaerobic respiration in this bacterium. The 83-kDa outer membrane cytochrome was purified and a specific polyclonal antibody was generated against this protein. Western blot analysis demonstrated that the vast majority of this protein was localized to the outer membrane and an intermediate density membrane fraction of similar composition. Its levels, but not its subcellular distribution, were somewhat influenced by the electron acceptor used to support anaerobic growth, with levels higher in fumarate-grown cells relative to iron(III)- or trimethylamine N-oxide-grown cells. Its specific content in cells grown under aerobic conditions was only 14% of that of fumarate-grown cells, suggesting that a switch to anaerobic conditions significantly increases the de novo synthesis of this outer membrane cytochrome.

Cloning and sequence of cymA, a gene encoding a tetraheme cytochrome c required for reduction of iron(III), fumarate, and nitrate by Shewanella putrefaciens MR-1

The cymA gene, which encodes a tetraheme cytochrome c, was cloned from Shewanella putrefaciens MR-1. This gene complemented a mutant which had a TnphoA insertion in cymA and which was deficient in the respiratory reduction of iron(III), nitrate, fumarate, and manganese(IV). The 561-bp nucleotide sequence of cymA encodes a protein of 187 amino acids with a predicted molecular mass of 20.8 kDa. No N-terminal signal sequence was readily apparent; consistent with this, a cytochrome with a size of 21 kDa was detected in the wild type but was absent in the insertional mutant. The cymA gene is transcribed into an mRNA; the major transcript was approximately 790 bases, suggesting that it is not part of a multicistronic operon. This RNA transcript was not detected in the cymA mutant. The CymA protein was found in the cytoplasmic membrane and soluble fraction of MR-1, and it shares partial amino acid sequence homology with multiheme c-type cytochromes from other bacteria. These cytochromes are ostensibly involved in the transfer of electrons from the cytoplasmic membrane to acceptors in the periplasm. The localization of the fumarate and iron(III) reductases to the periplasm and outer membrane of MR-1, respectively, suggests the possibility of a similar electron transfer role for CymA.

Localization of cytochromes to the outer membrane of anaerobically grown Shewanella putrefaciens MR-1

In gram-negative bacteria, numerous cell functions, including respiration-linked electron transport, have been ascribed to the cytoplasmic membrane. Gram-negative bacteria which use solid substrates (e.g., oxidized manganese or iron) as terminal electron acceptors for anaerobic respiration are presented with a unique problem: they must somehow establish an electron transport link across the outer membrane between large particulate metal oxides and the electron transport chain in the cytoplasmic membrane. When the metal-reducing bacterium Shewanella putrefaciens MR-1 is grown under anaerobic conditions and membrane fractions are purified from cells lysed by an EDTA-lysozyme-polyoxyethylene cetyl ether (Brij 58) protocol, approximately 80% of its membrane-bound cytochromes are localized in its outer membrane. These outer membrane cytochromes could not be dislodged by treatment with chaotropic agents or by increased concentrations of the nonionic detergent Brij 58, suggesting that they are integral membrane proteins. Cytochrome distribution in cells lysed by a French press protocol confirm the localization of cytochromes to the outer membrane of anaerobically grown cells. This novel cytochrome distribution could play a key role in the anaerobic respiratory capabilities of this bacterium, especially in its ability to mediate manganese and iron reduction.

Construction, characterization, and complementation of Rhodospirillum rubrum puf region mutants

Rhodospirillum rubrum is a facultatively phototrophic bacterium that, under certain growth conditions, forms an intracytoplasmic chromatophore membrane (ICM) housing the photochemical apparatus. The puf operon of R. rubrum encodes protein subunits of the photochemical reaction center and the B880 light-harvesting antenna complex. Mutant strains of R. rubrum were constructed by interposon mutagenesis through which a kanamycin resistance gene cartridge was inserted into restriction sites and in place of restriction fragments of the puf region. Southern blot analysis demonstrated that the defective copies of puf sequences had replaced their normal chromosomal counterparts through homologous recombination. The phenotypes of the mutant strains were evaluated on the basis of puf gene expression, spectral analysis, pigment content of membranes, and electron-microscopic examination of thin sections of cells grown under semi-aerobic and dark anaerobic conditions. Alterations of the puf region affect phototrophic competence and the formation of the ICM. The latter result implies an obligatory role for puf gene products in ICM formation in R. rubrum. One mutant with a deletion in puf structural genes was complemented in trans to the wild-type phenotype. Other mutants could be restored to the wild-type phenotype only by recombination.

Cell-cycle-specific fluctuation in cytoplasmic membrane composition in aerobically grown Rhodospirillum rubrum

Aerobic growth with synchronous cell division was induced in Rhodospirillum rubrum by starvation methods. Cells were harvested at different points in the cell cycle. Analysis of the composition of the cell envelope prepared by differential centrifugation or density gradient-purified cytoplasmic membrane obtained from cells at different times indicated that the protein/phospholipid ratio fluctuated with the cell cycle. The protein/phospholipid ratio of cell envelope from selection-synchronized cells also fluctuated with the cell cycle. These studies indicate that the phenomenon of cell-cycle-dependent fluctuation in membrane composition is not restricted to the intracytoplasmic chromatophore membrane of phototrophic cells.

Cell-cycle-specific oscillation in the composition of chromatophore membrane in Rhodospirillum rubrum

Synchrony in phototrophic cultures of Rhodospirillum rubrum was induced by stationary-phase cycling or by alterations in light intensity. Intracytoplasmic chromatophore membranes were prepared by differential centrifugation. Analysis of the composition of chromatophores obtained from cells at different times indicated that the protein/bacteriochlorophyll a ratio was constant throughout the cell cycle but that the protein/phospholipid ratio oscillated. This cell-cycle-dependent fluctuation in chromatophore membrane composition was reflected in the buoyant densities of the isolated chromatophores.

Succinate dehydrogenase in Rhodopseudomonas sphaeroides: subunit composition and immunocross-reactivity with other related bacteria

Antibodies were raised against the succinate dehydrogenase (SDH) present in the chromatophores of phototrophically grown Rhodopseudomonas sphaeroides. Crossed immunoelectrophoresis experiments indicated that the SDH present in the cytoplasmic membranes of heterotrophically grown R. sphaeroides is probably the same enzyme observed in the chromatophores. The enzyme was extracted by Triton X-100 in a form which consisted of only two subunits (molecular weight, 68,000 and 30,000) and was not associated with a cytochrome b. The antibodies directed against SDH from R. sphaeroides showed no immunocross-reactivity with SDH from phylogenetically related bacterial species, including Rhodopseudomonas capsulata, Paracoccus denitrificans, Rhodopseudomonas palustris, Rhodospirillum rubrum, and Rhodospirillum fulvum.