Switching of Swimming Modes in Magnetospirillium gryphiswaldense

The microaerophilic magnetotactic bacterium Magnetospirillum gryphiswaldense swims along magnetic field lines using a single flagellum at each cell pole. It is believed that this magnetotactic behavior enables cells to seek optimal oxygen concentration with maximal efficiency. We analyze the trajectories of swimming M. gryphiswaldense cells in external magnetic fields larger than the earth's field, and show that each cell can switch very rapidly (in <0.2 s) between a fast and a slow swimming mode. Close to a glass surface, a variety of trajectories were observed, from straight swimming that systematically deviates from field lines to various helices. A model in which fast (slow) swimming is solely due to the rotation of the trailing (leading) flagellum can account for these observations. We determined the magnetic moment of this bacterium using a to our knowledge new method, and obtained a value of (2.0±0.6) × 10(-16) A · m(2). This value is found to be consistent with parameters emerging from quantitative fitting of trajectories to our model.

Predicted structure and possible ionmotive mechanism of the sodium-linked NADH-ubiquinone oxidoreductase of Vibrio alginolyticus

Two groups have now published sequences of the six genes contained in the operon coding for the sodium-linked NADH-ubiquinone oxidoreductase of Vibrio alginolyticus. Sequence analyses indicate that this enzyme is unrelated to other known respiratory NADH dehydrogenases. A search for cofactor motifs suggests that the enzyme contains only one FAD, a ferredoxin-type iron sulphur centre, and the NADH-binding site. These are all located on NqrF, a subunit that can be recognized as a new member of a large diverse family of NAD(P)H-oxidizing flavoenzymes. A possible model of ion-coupling is presented, based upon this new information.

Spectroscopic and kinetic studies of the tetraheme flavocytochrome c from Shewanella putrefaciens NCIMB400

Electron paramagnetic resonance (EPR) and magnetic circular dichroism (MCD) spectroscopic studies were carried out on the tetraheme flavocytochrome c from Shewanella putrefaciens NCIMB400. The EPR spectrum reveals two sets of g-values--gz = 2.93, gy = 2.28, and gx = 1.51; and gz = 3.58--and the MCD spectrum shows a charge-transfer band at 1510 nm. These data combined show that all four hemes are low spin and have a nitrogenous sixth ligand. Sequence comparisons with other tetraheme cytochromes, particularly that from the purple phototroph H-1-R [Ambler, R.P. (1991) Biochem. Biophys. Acta 1058, 42-47], indicate that the sixth ligands are all histidines. Both the EPR data and the previously reported heme midpoint potentials [-220 and -320 mV; Morris, C.J., Black, A.C., Pealing, S.L., Manson, F.D.C., Chapman, S.K., Reid, G.A., Gibson, D.M., & Ward, F.B. (1994) Biochem. J. 302, 587-593] indicate that the hemes fall into two pairs. Stopped-flow kinetic experiments showed that fumarate-dependent heme oxidation was biphasic (kcat[fast] = 400 +/- 20 s-1; kcat[slow] = 34 +/- 3 s-1), with each phase having the same amplitude, confirming that the hemes are functionally paired.

Cloning and sequencing of four structural genes for the Na(+)-translocating NADH-ubiquinone oxidoreductase of Vibrio alginolyticus

Oligonucleotide probes based on the N-terminal amino acid sequences of the NqrA and NqrC subunits were used to clone genes for the Na(+)-dependent NADH-ubiquinone oxidoreductase complex from Vibrio alginolyticus. Four consecutive ORFs were identified encoding subunit proteins of 48.6, 46.8, 27.7 and 22.6 kDa, respectively (NqrA-D). A further ORF, showing 71% homology to the BolA protein of Escherichia coli, was located upstream. From sequence comparisons, we conclude that the Na(+)-dependent NADH-ubiquinone oxidoreductase complex of V. alginolyticus is clearly distinct from the corresponding H(+)-dependent enzymes of both prokaryotes and eukaryotes.

Purification and properties of a novel cytochrome: flavocytochrome c from Shewanella putrefaciens

The major soluble cytochrome isolated from microaerobically grown cells of Shewanella putrefaciens has been shown to be a novel type of flavocytochrome with fumarate reductase activity. This flavocytochrome, located in the periplasmic fraction of cell extracts, has been purified to homogeneity and shown to contain 4 mol of haem c and 1 mol of non-covalently bound FAD per mol of protein. An M(r) value of 63,800 is estimated from sequence analysis assuming 4 mol of haem/mol of protein. In the presence of the artificial electron donor, reduced methyl viologen, the flavocytochrome catalysed the reduction of fumarate but not that of nitrite, dimethylsulphoxide, trimethylamine-N-oxide or sulphite. The pH optimum was 7.4 with calculated pKa values of 6.8 and 8.0 for contributing catalytic groups. The Km and kcat. values for fumarate reduction were 21 microM and 250 s-1 respectively, whereas the corresponding values for succinate oxidation with 2,6-dichlorophenol-indophenol as electron carriers were 200 microM and 0.07 s-1 respectively. Mesaconic acid was a competitive inhibitor of fumarate reduction with a Ki of 2 microM. Zymogram staining of polyacrylamide gels with purified protein showed a band of fumarate reductase activity. Polyclonal antibodies, raised to the purified flavocytochrome, were shown to titrate out fumarate reductase activity. We conclude that the physiological role of this enzyme is as a fumarate reductase. Optical absorption spectra of the flavocytochrome indicated that all the haems were of the c-type and gave alpha, beta and gamma peaks at 552.3, 523 and 418 nm in the reduced spectrum with epsilon values of 30.2, 15.9 and 188.2 mM-1.cm-1 respectively. Oxidized spectra showed no 695 nm band that would be indicative of His-Met coordination. Two redox potentials were resolved at -220 mV and -320 mV. The cytochrome was reduced by formate in the presence of particulate cell fractions. The relationship of this cytochrome to other low-potential flavocytochromes c is discussed.

Sequence of the gene encoding flavocytochrome c from Shewanella putrefaciens: a tetraheme flavoenzyme that is a soluble fumarate reductase related to the membrane-bound enzymes from other bacteria

Flavocytochrome c from the Gram-negative, food-spoiling bacterium Shewanella putrefaciens is a soluble, periplasmic fumarate reductase. We have isolated the gene encoding flavocytochrome c and determined the complete DNA sequence. The predicted amino acid sequence indicates that flavocytochrome c is synthesized with an N-terminal secretory signal sequence of 25 amino acid residues. The mature protein contains 571 amino acid residues and consists of an N-terminal cytochrome domain, of about 117 residues, with four heme attachment sites typical of c-type cytochromes and a C-terminal flavoprotein domain of about 454 residues that is clearly related to the flavoprotein subunits of fumarate reductases and succinate dehydrogenases from bacterial and other sources. A second reading frame that may be cotranscribed with the flavocytochrome c gene exhibits some similarity with the 13-kDa membrane anchor subunit of Escherichia coli fumarate reductase. The sequence of the flavoprotein domain demonstrates an even closer relationship with the product of the yeast OSM1 gene, mutations in which result in sensitivity to high osmolarity. These findings are discussed in relation to the function of flavocytochrome c.

Influence of respiratory substrate on the cytochrome content of Shewanella putrefaciens

Shewanella putrefaciens can use trimethylamine oxide (TMAO) as electron acceptor under anoxic conditions. The associated cytochromes induced during growth under various respiratory conditions have been separated by liquid chromatography (DEAE Sepharose CL6b) and SDS-PAGE and characterized spectrophotometrically and by redox potentiometry. Two major low potential cytochromes and at least three minor low potential cytochromes, likely to be involved in TMAO reduction, were found. No cytochrome specific for TMAO reductase was found.

Analysis of tryptic digests of bovine beta-casein by reversed-phase high-performance liquid chromatography

Reversed-phase high-performance liquid chromatography (RP-HPLC) was used to separate the tryptic peptides of beta-casein. A gradient of 0-50% acetonitrile in 0.1% trifluoroacetic acid at a flow-rate of 0.8 ml/min resolved ten of the thirteen peptides. A modified gradient resolved the three peptides eluted at ca. 25% acetonitrile. RP-HPLC proved superior to high-voltage paper electrophoresis in analysis time, resolution and flexibility. The methods developed for the analysis of proteolysis of the milk protein, beta-casein, are now being applied to study the action of extracellular proteases from dairy bacteria on milk proteins.

Enzyme immunoassay for free thyroxin

We describe a mathematical model for a single-tube enzyme immunoassay for free thyroxin (FT4), involving use of a thyroxin/horseradish peroxidase (EC 1.11.1.7) conjugate that does not interact with thyroxin-binding globulin. In the presence of serum two populations of unassociated, or free, immunologically active constituents are present: FT4 and the conjugate. The concentrations of the former are determined by the serum constituents and of the latter by the albumin concentration. When a small quantity of antibody is added, it reacts with the variable amount of FT4 and with the constant amount of the conjugate, thus giving a measure of the FT4. We constructed a mathematical model based on thermodynamic binding constants and adsorption data. The model gives satisfactory agreement with the experimental data under a variety of experimental conditions. Results for 19 patients' serum samples demonstrate the validity of the concept.

Enzyme immunoassay for free thyroxin

We describe a mathematical model for a single-tube enzyme immunoassay for free thyroxin (FT4), involving use of a thyroxin/horseradish peroxidase (EC 1.11.1.7) conjugate that does not interact with thyroxin-binding globulin. In the presence of serum two populations of unassociated, or free, immunologically active constituents are present: FT4 and the conjugate. The concentrations of the former are determined by the serum constituents and of the latter by the albumin concentration. When a small quantity of antibody is added, it reacts with the variable amount of FT4 and with the constant amount of the conjugate, thus giving a measure of the FT4. We constructed a mathematical model based on thermodynamic binding constants and adsorption data. The model gives satisfactory agreement with the experimental data under a variety of experimental conditions. Results for 19 patients' serum samples demonstrate the validity of the concept.