Gene to ultrastructure: the case of the flagellar basal body

Motility and chemotaxis in alkaliphilic Bacillus species

Alkaliphilic Bacillus species grow at pH values up to approximately 11. Motile alkaliphilic Bacillus use electrochemical gradients of Na(+) (sodium-motive force) to power ion-coupled, flagella-mediated motility as opposed to the electrochemical gradients of H(+) (proton-motive force) used by most neutralophilic bacteria. Membrane-embedded stators of bacterial flagella contain ion channels through which either H(+) or Na(+) flow to energize flagellar rotation. Stators of the major H(+)-coupled type, MotAB, are distinguishable from Na(+)-coupled stators, PomAB of marine bacteria and MotPS of alkaliphilic Bacillus. Dual ion-coupling capacity is found in neutralophilic Bacillus strains with both MotAB and MotPS. There is also a MotAB variant that uses both coupling ions, switching as a function of pH. Chemotaxis of alkaliphilic Bacillus depends upon flagellar motility but also requires a distinct voltage-gated NaChBac-type channel. The two alkaliphile Na(+) channels provide new vistas on the diverse adaptations of sensory responses in bacteria.

Na(+)-driven flagellar motor of Vibrio

Bacterial flagellar motors are molecular machines powered by the electrochemical potential gradient of specific ions across the membrane. Bacteria move using rotating helical flagellar filaments. The flagellar motor is located at the base of the filament and is buried in the cytoplasmic membrane. Flagellar motors are classified into two types according to the coupling ion: namely the H(+)-driven motor and the Na(+)-driven motor. Analysis of the flagellar motor at the molecular level is far more advanced in the H(+)-driven motor than in the Na(+)-driven motor. Recently, the genes of the Na(+)-driven motor have been cloned from a marine bacterium of Vibrio sp. and some of the motor proteins have been purified and characterized. In this review, we summarize recent studies of the Na(+)-driven flagellar motor.

The fliU and fliV genes are expressed as a single ORF in Salmonella choleraesuis

A DNA fragment carrying flagellar genes was cloned from Salmonella choleraesuis. Compared to the corresponding DNA fragment of Salmonella muenchen, this fragment contained three ORFs instead of four shown in S. muenchen. The DNA sequence data showed that there was an insertion of nucleotide C in the ORF of the S. choleraesuis fliU gene, which resulted in the disappearance of a termination codon downstream. The recombinant plasmid pFU11 containing the coding region of the fliU gene made by PCR on S. choleraesuis genomic DNA was constructed and expressed in Escherichia coli in the presence of IPTG. As expected, a 45 kDa protein band was observed on a SDS-PAGE gel, in contrast to two with each having about a half of the molecular weight. These results demonstrated that the DNA sequence encoding one protein (FliU) in S. choleraesuis corresponded to the DNA sequence encoding two proteins (FliU and FliV) in S. muenchen. The protein encoded by this single ORF might carry out the functions of two separated proteins by folding in such a way that its conformation could function like two interdependent protein subunits.

Rotational asymmetry of Escherichia coli flagellar motor in the presence of arsenate

The flagellar motor of Escherichia coli (E. coli) is driven by a proton-motive force (PMF), hence it was of interest to determine whether the motor is symmetrical in the sense that it can be rotated by any polarity of PMF. For this purpose the cells had to be deenergized first. Conventional deenergization procedures caused irreversible loss of motility, presumably due to ATP-dependent degradative processes. However, E. coli cells deenergized by incubation with arsenate manifested a slow, reversible depletion of PMF. In this procedure there was a sufficiently long time window, during which a considerable proportion of the cells lost their motility and could be made to rotate again by an artificially-imposed PMF. The motors of these cells rotated in response to any PMF polarity, but positive and negative polarities rotated different sub-populations of cells and the direction was almost exclusively counterclockwise. The reason for the unidirectionality of the rotation was not the intervention of the chemotaxis system. A number of potential reasons are suggested. One is the arsenate effect on the motor function found previously [Margolin, Y., Barak, R. and Eisenbach, M. (1994) J. Bacteriol. 176, 5547-5549]. A possible interaction between arsenate and the motor is discussed.

Conserved machinery of the bacterial flagellar motor

Novel periplasmic and cytoplasmic structural modules of the bases of bacterial flagella have been observed in situ and isolated using new biochemical protocols. Flagellar rotation may depend upon interactions of these modules with the intramembrane particle rings, a ubiquitous feature of flagellar bases necessary for torque generation. The outer membrane-associated basal disk of the Wolinella succinogenes polar flagellum has architecture well suited for interaction with the ring particles. However, antibody against the main W. succinogenes basal disk protein did not cross-react with flagella-enriched fractions from Salmonella typhimurium and Bacillus firmus; nor have such structures been observed in these species thus far. Antibodies against two S. typhimurium proteins, FliG and FliM, known to be involved in motor function and part of the cytoplasmic module in this species cross-reacted with flagella-enriched fractions from both W. succinogenes and B. firmus. In addition, flagellar cytoplasmic structure could be isolated from B. firmus. The basal disk may anchor the flagellar motor to the cell wall in some polar bacteria, but this does not seem to be a unique strategy. In contrast, the data indicate that the cytoplasmic module is conserved.

Chemiosmotic concept of the membrane bioenergetics: what is already clear and what is still waiting for elucidation?

The present state of the chemiosmotic concept is reviewed. Special attention is paid to (i) further progress in studies on the Na(+)-coupled energetics and (ii) paradoxical bioenergetic effects when protonic or sodium potentials are utilized outside the coupling membrane (TonB-mediated uphill transports across the outer bacterial membrane). A hypothesis is put forward assuming that the same principle is employed in the bacterial flagellar motor.

Physical characterization of the flagella and flagellins from Methanospirillum hungatei

Flagellar filaments from Methanospirillum hungatei GP1 and JF1 were isolated and subjected to a variety of physical and chemical treatments. The filaments were stable to temperatures up to 80 degrees C and over the pH range of 4 to 10. The flagellar filaments were dissociated in the detergents (final concentration of 0.5%) Triton X-100, Tween 20, Tween 80, Brij 58, N-octylglucoside, cetyltrimethylammonium bromide, and Zwittergent 3-14, remaining intact in only two of the detergents tested, sodium deoxycholate and 3-[(3-cholamidopropyl)-dimethyl-ammonio]-1-propanesulfonate (CHAPS). Spheroplasting techniques were used to separate the internal cells from the complex sheath, S-layer (cell wall), and end plugs of M. hungatei. The flagellar basal structure was visualized after solubilization of membranes by CHAPS or deoxycholate. The basal structure appeared to be a simple knob with no apparent ring or hook structures. The multiple, glycosylated flagellins constituting the flagellar filaments were cleaved by proteases and cyanogen bromide. The cyanogen bromide-generated fragments of M. hungatei GP1 flagellins were partially sequenced to provide internal sequence information. In addition, the amino acid composition of each flagellin was determined and indicated that the flagellins are distinct gene products, rather than differentially glycosylated forms of the same gene product.

Nucleotide sequence of the Wolinella succinogenes flagellin, which contains in the antigenic domain two conserved regions also present in Campylobacter spp. and Helicobacter pylori

Wolinella succinogenes possesses one polar flagellum, which shows a characteristic surface pattern of parallel lines along the axis of the filament in electron microscopic images. We determined the gene sequence of the Wolinella flagellin, which is, as in most other bacteria, the only structural component of the filament. Sequence comparison with other members of the Proteobacteria revealed two highly conserved regions in the central part of the flagellin molecule among Campylobacter spp. and Helicobacter pylori, an area that had previously been described as highly variable. Similar surface patterns are found in related polarly flagellated bacteria, but not in Escherichia coli and Bacillus subtilis, which also lack these conserved regions.

Bioenergetics: the evolution of molecular mechanisms and the development of bioenergetic concepts

Possible routes for the evolution of cell energetics are considered. It is assumed that u.v. light was the primary energy source for the precursors of the primordial living cell and that primitive energetics might have been based on the use of the adenine moiety of ADP as the u.v. chromophore. It is proposed that the excitation of the adenine residue facilitated phosphorylation of its amino group with subsequent transfer of a phosphoryl group to the terminal phosphate of ADP to form ATP. ATP-driven carbohydrate synthesis is considered as a mechanism for storing u.v.-derived energy, which was then used in the dark. Glycolysis presumably produced compounds like ethanol and CO2, which easily penetrate the membrane and therefore were lost by the cell. Later lactate-producing glycolysis appeared, the end product being non-penetrant and, hence, retained inside the cell to be utilized to regenerate carbohydrates when light energy became available. Production of lactate was accompanied by accumulation of equimolar H+. To avoid acidification of the cell interior, an F0-type H+ channel was employed. Later it was supplemented with F1. This allowed the ATP energy to be used for 'uphill' H+ pumping to the medium, which was acidified due to glycolytic activity of the cells. In the subsequent course of evolution, u.v. light was replaced by visible light, which has lower energy but is less dangerous for the cell. It is assumed that bacteriorhodopsin, a simple and very stable light-driven H+ pump which still exists in halophilic and thermophilic Archaea, was the primary system utilizing visible light. The delta mu-H+ formed was used to reverse the H(+)-ATPase, which began to function as H(+)-ATP-synthase. Later, bacteriorhodopsin photosynthesis was substituted by a more efficient chlorophyll photosynthesis, producing not only ATP, but also carbohydrates. O2, a side product of this process, was consumed by the H(+)-motive respiratory chain to form delta mu-H+ in the dark. At the next stage of evolution, a parallel energy-transducing mechanism appeared which employed Na+ instead of H+ as the coupling ion (the Na+ cycle).(ABSTRACT TRUNCATED AT 400 WORDS)