Laccases are widespread in bacteria.
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Bacteria↗
Biomedical subjects
I B Zhulin
Publications and source records attributed to I B Zhulin.
Loss of cytochrome c oxidase activity and acquisition of resistance to quinone analogs in a laccase-positive variant of Azospirillum lipoferum.
Laccase, a p-diphenol oxidase typical of plants and fungi, has been found recently in a proteobacterium, Azospirillum lipoferum. Laccase activity was detected in both a natural isolate and an in vitro-obtained phase variant that originated from the laccase-negative wild type. In this study, the electron transport systems of the laccase-positive variant and its parental laccase-negative forms were compared. During exponential (but not stationary) growth under fully aerobic (but not under microaerobic) conditions, the laccase-positive variant lost a respiratory branch that is terminated in a cytochrome c oxidase of the aa(3) type; this was most likely due to a defect in the biosynthesis of a heme component essential for the oxidase. The laccase-positive variant was significantly less sensitive to the inhibitory action of quinone analogs and fully resistant to inhibitors of the bc(1) complex, apparently due to the rearrangements of its respiratory system. We propose that the loss of the cytochrome c oxidase-containing branch in the variant is an adaptive strategy to the presence of intracellular oxidized quinones, the products of laccase activity.
Azospirillum↗
PAS domains: internal sensors of oxygen, redox potential, and light.
PAS domains are newly recognized signaling domains that are widely distributed in proteins from members of the Archaea and Bacteria and from fungi, plants, insects, and vertebrates. They function as input modules in proteins that sense oxygen, redox potential, light, and some other stimuli. Specificity in sensing arises, in part, from different cofactors that may be associated with the PAS fold. Transduction of redox signals may be a common mechanistic theme in many different PAS domains. PAS proteins are always located intracellularly but may monitor the external as well as the internal environment. One way in which prokaryotic PAS proteins sense the environment is by detecting changes in the electron transport system. This serves as an early warning system for any reduction in cellular energy levels. Human PAS proteins include hypoxia-inducible factors and voltage-sensitive ion channels; other PAS proteins are integral components of circadian clocks. Although PAS domains were only recently identified, the signaling functions with which they are associated have long been recognized as fundamental properties of living cells.
Amino Acid Sequence↗
Aerotaxis and other energy-sensing behavior in bacteria.
Energy taxis is widespread in motile bacteria and in some species is the only known behavioral response. The bacteria monitor their cellular energy levels and respond to a decrease in energy by swimming to a microenvironment that reenergizes the cells. This is in contrast to classical Escherichia coli chemotaxis in which sensing of stimuli is independent of cellular metabolism. Energy taxis encompasses aerotaxis (taxis to oxygen), phototaxis, redox taxis, taxis to alternative electron acceptors, and chemotaxis to a carbon source. All of these responses share a common signal transduction pathway. An environmental stimulus, such as oxygen concentration or light intensity, modulates the flow of reducing equivalents through the electron transport system. A transducer senses the change in electron transport, or possibly a related parameter such as proton motive force, and initiates a signal that alters the direction of swimming. The Aer and Tsr proteins in E. coli are newly recognized transducers for energy taxis. Aer is homologous to E. coli chemoreceptors but unique in having a PAS domain and a flavin-adenine dinucleotide cofactor that is postulated to interact with a component of the electron transport system. PAS domains are energy-sensing modules that are found in proteins from archaea to humans. Tsr, the serine chemoreceptor, is an independent transducer for energy taxis, but its sensory mechanism is unknown. Energy taxis has a significant ecological role in vertical stratification of microorganisms in microbial mats and water columns. It plays a central role in the behavior of magnetotactic bacteria and also appears to be important in plant-microbe interactions.
Bacterial Physiological Phenomena↗
In search of higher energy: metabolism-dependent behaviour in bacteria.
Bacteria use different strategies to navigate to niches where environmental factors are favourable for growth. Chemotaxis is a behavioural response mediated by specific receptors that sense the concentration of chemicals in the environment. Recently, a new type of sensor has been described in Escherichia coli that responds to changes in cellular energy (redox) levels. This sensor, Aer, guides the bacteria to environments that support maximal energy levels in the cells. A variety of stimuli, such as oxygen, alternative electron acceptors, light, redox carriers that interact with the electron transport system and metabolized carbon sources, effect changes in the cellular energy (redox) levels. These changes are detected by Aer and by the serine chemotaxis receptor Tsr and are transduced into signals that elicit appropriate behavioural responses. Diverse environmental signals from Aer and chemotaxis receptors converge and integrate at the level of the CheA histidine kinase. Energy sensing is widespread in bacteria, and it is now evident that a variety of signal transduction strategies are used for the metabolism-dependent behaviours. The occurrence of putative energy-sensing domains in proteins from cells ranging from Archaea to humans indicates the importance of this function for all living systems.
Bacterial Physiological Phenomena↗
Correlation of PAS domains with electron transport-associated proteins in completely sequenced microbial genomes.
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Archaeal Proteins↗
The Aer protein and the serine chemoreceptor Tsr independently sense intracellular energy levels and transduce oxygen, redox, and energy signals for Escherichia coli behavior.
We identified a protein, Aer, as a signal transducer that senses intracellular energy levels rather than the external environment and that transduces signals for aerotaxis (taxis to oxygen) and other energy-dependent behavioral responses in Escherichia coli. Domains in Aer are similar to the signaling domain in chemotaxis receptors and the putative oxygen-sensing domain of some transcriptional activators. A putative FAD-binding site in the N-terminal domain of Aer shares a consensus sequence with the NifL, Bat, and Wc-1 signal-transducing proteins that regulate gene expression in response to redox changes, oxygen, and blue light, respectively. A double mutant deficient in aer and tsr, which codes for the serine chemoreceptor, was negative for aerotaxis, redox taxis, and glycerol taxis, each of which requires the proton motive force and/or electron transport system for signaling. We propose that Aer and Tsr sense the proton motive force or cellular redox state and thereby integrate diverse signals that guide E. coli to environments where maximal energy is available for growth.
Amino Acid Sequence↗
Terminal oxidases of Azoarcus sp. BH72, a strictly respiratory diazotroph.
The nitrogen-fixing, grass-associated bacterium Azoarcus sp. BH72 was characterized with respect to its terminal oxidases. Inhibitory respiratory analysis revealed the presence of at least one cytochrome c oxidase and one quinol oxidase. The cytochrome c oxidase was preferably used by the cells under aerobic, whereas the quinol oxidase seemed to be dominant under microaerobic, nitrogen-fixing conditions. Differential spectroscopy and heme analysis of the membrane preparations indicated that the cytochrome c oxidase is probably of the cb type.
Chromatography, High Pressure Liquid↗
PAS domain S-boxes in Archaea, Bacteria and sensors for oxygen and redox.
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Amino Acid Sequence↗
How do bacteria avoid high oxygen concentrations?
Bacteria, such as Escherichia coli and Azospirillum brasilense, avoid microenvironments with elevated oxygen concentrations, not by sensing reactive oxygen derivatives, but by sensing a metabolic down-shift that results from elevated oxygen levels. A novel protein, Aer, and the chemotaxis serine receptor, Tsr, have recently been identified as transducers for aerotaxis which monitor internal energy levels in the bacteria.
Bacterial Physiological Phenomena↗
Glycerol elicits energy taxis of Escherichia coli and Salmonella typhimurium.
Escherichia coli and Salmonella typhimurium show positive chemotaxis to glycerol, a chemical previously reported to be a repellent for E. coli. The threshold of the attractant response in both species was 10(-6) M glycerol. Glycerol chemotaxis was energy dependent and coincident with an increase in membrane potential. Metabolism of glycerol was required for chemotaxis, and when lactate was present to maintain energy production in the absence of glycerol, the increases in membrane potential and chemotactic response upon addition of glycerol were abolished. Methylation of a chemotaxis receptor was not required for positive glycerol chemotaxis in E. coli or S. typhimurium but is involved in the negative chemotaxis of E. coli to high concentrations of glycerol. We propose that positive chemotaxis to glycerol in E. coli and S. typhimurium is an example of energy taxis mediated via a signal transduction pathway that responds to changes in the cellular energy level.
Bacterial Proteins↗
Oxygen-dependent growth of the obligate anaerobe Desulfovibrio vulgaris Hildenborough.
Desulfovibrio vulgaris Hildenborough, a sulfate-reducing bacterium classified as an obligate anaerobe, swam to a preferred oxygen concentration of 0.02 to 0.04% (0.24 to 0.48 microM), a level which also supported growth. Oxygen concentrations of 0.08% and higher arrested growth. We propose that in zones of transition from an oxic to an anoxic environment, D. vulgaris protects anoxic microenvironments from intrusion of oxygen.
Aerobiosis↗
Behavioral responses of Escherichia coli to changes in redox potential.
Escherichia coli bacteria sensed the redox state in their surroundings and they swam to a niche that had a preferred reduction potential. In a spatial redox gradient of benzoquinone/benzoquinol, E. coli cells migrated to form a sharply defined band. Bacteria swimming out of either face of the band tumbled and returned to the preferred conditions at the site of the band. This behavioral response was named redox taxis. Redox molecules, such as substituted quinones, that elicited redox taxis, interact with the bacterial electron transport system, thereby altering electron transport and the proton motive force. The magnitude of the behavioral response was dependent on the reduction potential of the chemoeffector. The Tsr, Tar, Trg, Tap, and CheR proteins, which have a role in chemotaxis, were not essential for redox taxis. A cheB mutant had inverted responses in redox taxis, as previously demonstrated in aerotaxis. A model is proposed in which a redox effector molecule perturbs the electron transport system, and an unknown sensor in the membrane detects changes in the proton motive force or the redox status of the electron transport system, and transduces this information into a signal that regulates phosphorylation of the CheA protein. A similar mechanism has been proposed for aerotaxis. Redox taxis may play an important role in the distribution of bacterial species in natural environments.
Bacterial Proteins↗
Oxygen taxis and proton motive force in Azospirillum brasilense.
The microaerophilic nitrogen-fixing bacterium Azospirillum brasilense formed a sharply defined band in a spatial gradient of oxygen. As a result of aerotaxis, the bacteria were attracted to a specific low concentration of oxygen (3 to 5 microM). Bacteria swimming away from the aerotactic band were repelled by the higher or lower concentration of oxygen that they encountered and returned to the band. This behavior was confirmed by using temporal gradients of oxygen. The cellular energy level in A. brasilense, monitored by measuring the proton motive force, was maximal at 3 to 5 microM oxygen. The proton motive force was lower at oxygen concentrations that were higher or lower than the preferred oxygen concentration. Bacteria swimming toward the aerotactic band would experience an increase in the proton motive force, and bacteria swimming away from the band would experience a decrease in the proton motive force. It is proposed that the change in the proton motive force is the signal that regulates positive and negative aerotaxis. The preferred oxygen concentration for aerotaxis was similar to the preferred oxygen concentration for nitrogen fixation. Aerotaxis is an important adaptive behavioral response that can guide these free-living diazotrophs to the optimal niche for nitrogen fixation in the rhizosphere.
Azospirillum brasilense↗
Behavior of Rhizobium meliloti in oxygen gradients.
Rhizobium meliloti cells responded to an abrupt change in oxygen concentration by changing the cell speed (chemokinesis), but they did not alter the frequency at which swimming cells stopped briefly (aerotaxis). Changes in cell speed upon stimulation with oxygen coincided with changes in membrane potential. The cells did not form an aerotactic band in a spatial gradient of oxygen as do the cells of other bacterial species. The fixL and fixJ genes which encode a heme-containing protein kinase that senses oxygen and a response regulator, respectively, were not involved in the behavior of R. meliloti in oxygen gradients.
Bacterial Proteins↗
Role of methylation in aerotaxis in Bacillus subtilis.
Taxis to oxygen (aerotaxis) in Bacillus subtilis was characterized in a capillary assay and in a temporal assay in which the concentration of oxygen in a flow chamber was changed abruptly. A strong aerophilic response was present, but there was no aerophobic response to high concentrations of oxygen. Adaptation to a step increase in oxygen concentration was impaired when B. subtilis cells were depleted of methionine to prevent methylation of the methyl-accepting chemotaxis proteins. There was a transient increase in methanol release when wild-type B. subtilis, but not a cheR mutant that was deficient in methyltransferase activity, was stimulated by a step increase or a step decrease in oxygen concentration. The methanol released was quantitatively correlated with demethylation of methyl-accepting chemotaxis proteins. This indicated that methylation is involved in aerotaxis in B. subtilis in contrast to aerotaxis in Escherichia coli and Salmonella typhimurium, which is methylation independent.
Aerobiosis↗
Motility, chemokinesis, and methylation-independent chemotaxis in Azospirillum brasilense.
Observations of free-swimming and antibody-tethered Azospirillum brasilense cells showed that their polar flagella could rotate in both clockwise and counterclockwise directions. Rotation in a counterclockwise direction caused forward movement of free-swimming cells, whereas the occasional change in the direction of rotation to clockwise caused a brief reversal in swimming direction. The addition of a metabolizable chemoattractant, e.g., malate or proline, had two distinct effects on the swimming behavior of the bacteria: (i) a short-term decrease in reversal frequency from 0.33 to 0.17 s-1 and (ii) a long-term increase in the mean population swimming speed from 13 to 23 microns s-1. A. brasilense therefore shows both chemotaxis and chemokinesis in response to temporal gradients of some chemoeffectors. Chemokinesis was dependent on the growth state of the cells and may depend on an increase in the electrochemical proton gradient above a saturation threshold. Analysis of behavior of a methionine auxotroph, assays of in vivo methylation, and the use of specific antibodies raised against the sensory transducer protein Tar of Escherichia coli all failed to demonstrate the methylation-dependent pathway for chemotaxis in A. brasilense. The range of chemicals to which A. brasilense shows chemotaxis and the lack of true repellents indicate an alternative chemosensory pathway probably based on metabolism of chemoeffectors.
Azospirillum brasilense↗