Roles of chemosensory pathways in transient changes in swimming speed of Rhodobacter sphaeroides induced by changes in photosynthetic electron transport

Light-controlled motility in prokaryotes and the problem of directional light perception

The natural light environment is important to many prokaryotes. Most obviously, phototrophic prokaryotes need to acclimate their photosynthetic apparatus to the prevailing light conditions, and such acclimation is frequently complemented by motility to enable cells to relocate in search of more favorable illumination conditions. Non-phototrophic prokaryotes may also seek to avoid light at damaging intensities and wavelengths, and many prokaryotes with diverse lifestyles could potentially exploit light signals as a rich source of information about their surroundings and a cue for acclimation and behavior. Here we discuss our current understanding of the ways in which bacteria can perceive the intensity, wavelength and direction of illumination, and the signal transduction networks that link light perception to the control of motile behavior. We discuss the problems of light perception at the prokaryotic scale, and the challenge of directional light perception in small bacterial cells. We explain the peculiarities and the common features of light-controlled motility systems in prokaryotes as diverse as cyanobacteria, purple photosynthetic bacteria, chemoheterotrophic bacteria and haloarchaea.

A minimal model for metabolism-dependent chemotaxis in Rhodobacter sphaeroides (†)

Chemotaxis is vital cellular movement in response to environmental chemicals. Unlike the canonical chemotactic pathway in Escherichia coli, Rhodobacter sphaeroides has both transmembrane and cytoplasmic sensory clusters, with the latter possibly interacting with essential components in the electron transport system. However, the effect of the cytoplasmic sensor and the mechanism of signal integration from both sensory clusters remain unclear. Based on a minimal model of the chemotaxis pathway in this species, we show that signal integration at the motor level produces realistic chemotactic behaviour in line with experimental observations. Our model also suggests that the core pathway of R. sphaeroides, at least its ancestor, may represent a metabolism-dependent selective stopping strategy, which alone can steer cells to favourable environments. Our results not only clarify the potential roles of the two sensory clusters but also put in question the current definitions of attractants and repellents.

Soluble electron shuttles can mediate energy taxis toward insoluble electron acceptors

Shewanella species grow in widely disparate environments and play key roles in elemental cycling, especially in environments with varied redox conditions. To obtain a system-level understanding of Shewanella's robustness and versatility, the complex interplay of cellular growth, metabolism, and transport under conditions of limiting carbon sources, energy sources, and electron acceptors must be elucidated. In this paper, population-level taxis of Shewanella oneidensis MR-1 cells in the presence of a rate-limiting, insoluble electron acceptor was investigated. A novel mechanism, mediated energy taxis, is proposed by which Shewanella use riboflavin as both an electron shuttle and an attractant to direct cell movement toward local sources of insoluble electron acceptors. The cells secrete reduced riboflavin, which diffuses to a nearby particle containing an insoluble electron acceptor and is oxidized. The oxidized riboflavin then diffuses away from the particle, establishing a spatial gradient that draws cells toward the particle. Experimental and modeling results are presented to support this mechanism. S. oneidensis MR-1 cells inoculated into a uniform dispersion of MnO(2) particles in dilute agar exhibited taxis outward, creating a clear zone within which riboflavin was detected by mass spectrometry. Cells inoculated into dilute agar containing oxidized riboflavin similarly exhibited taxis, rapidly forming an expanding zone of reduced riboflavin. A mathematical model based on the proposed mechanism was able to predict experimental trends, including how concentrations of riboflavin and insoluble electron acceptors (e.g., MnO(2)) affected tactic cell migration.

Comparison of aerobic and photosynthetic Rhodobacter sphaeroides 2.4.1 proteomes

The analysis of proteomes from aerobic and photosynthetic Rhodobacter sphaeroides 2.4.1 cell cultures by liquid chromatography-mass spectrometry yielded approximately 6,500 high confidence peptides representing 1,675 gene products (39% of the predicted proteins). The identified proteins corresponded primarily to open reading frames (ORFs) contained within the two chromosomal elements of this bacterium, but a significant number were also observed from ORFs associated with 5 naturally occurring plasmids. Using the accurate mass and time (AMT) tag approach, comparative studies showed that a number of proteins were uniquely detected within the photosynthetic cell culture. The estimated abundances of proteins observed in both aerobic respiratory and photosynthetic grown cultures were compared to provide insights into bioenergetic models for both modes of growth. Additional emphasis was placed on gene products annotated as hypothetical to gain information as to their potential roles within these two growth conditions. Where possible, transcriptome and proteome data for R. sphaeroides obtained under the same culture conditions were also compared.

Light-dependent regulation of photosynthesis genes in Rhodobacter sphaeroides 2.4.1 is coordinately controlled by photosynthetic electron transport via the PrrBA two-component system and the photoreceptor AppA

Formation of the photosynthetic apparatus in Rhodobacter is regulated by oxygen tension and light intensity. Here we show that in anaerobically grown Rhodobacter cells a light-dependent increase in expression of the puc and puf operons encoding structural proteins of the photosynthetic complexes requires an active photosynthetic electron transport. The redox-sensitive CrtJ/PpsR repressor of photosynthesis genes, which was suggested to mediate electron transport-dependent signals, is not involved in this light-dependent signal chain. Our data reveal that the signal initiated in the photosynthetic reaction centre is transmitted via components of the electron transport chain and the PrrB/PrrA two-component system in Rhodobacter sphaeroides. Under blue light illumination in the absence of oxygen this signal leads to activation of photosynthesis genes and interferes with a blue-light repression mediated by the AppA photoreceptor and the PpsR transcriptional repressor in R. sphaeroides. Thus, light either sensed by a photoreceptor or initiating photosynthetic electron transport has opposite effects on the transcription of photosynthesis genes. Both signalling pathways involve redox-dependent steps that finally determine the effect of light on gene expression.

Blue light perception in bacteria

This review focuses on the blue light responses in bacteria and on the bacterial proteins which have been demonstrated to function as blue light receptors. Results of the previous years reveal that different types of photoreceptors have already evolved in prokaryotes. However, for most of these photoreceptors the exact biological functions and the mechanisms of signaling to downstream components are poorly understood.

Fed-batch culture of Bacillus thuringiensis based on motile intensity

The operation of a fed-batch culture is more complicated than that of batch or continuous culture. Thus, an appropriate feeding strategy for fed-batch cultures should be carefully designed. In this study, a simple feeding strategy for fed-batch culture of Bacillus thuringiensis based on "motile intensity" is described. The feeding strategy consisted of two steps: (1) initiating feeding at the peak of motile intensity; (2) terminating feeding at low motile intensity (or non-motility) of the cells. In addition, the motile intensity of B. thuringiensis was used to determine the optimum environmental conditions (pH, temperature, and dissolved oxygen) and optimum medium composition. Using this fed-batch strategy, the production of thuringiensin increased 34% compared with batch culture using the same environmental conditions and medium composition. The proposed strategy for fed-batch culture helps to avoid overfeeding of substrate and facilitates on-line control. A comparison of several alternative strategies for fed-batch culture demonstrated that strategies such as glucose-stat and DO-stat result in a lower productivity than that obtained using the motility intensity method.

Light-induced behavioral responses (;phototaxis') in prokaryotes

Light-induced sensory responses are among the oldest scientific observations on bacterial behavior. Various types of response have been characterized physiologically in detail. However, the molecular basis of this type of response is only slowly emerging. In many of these systems photosynthetic pigments absorb the light. This then generates a signal via electron transport, feeding into a canonical chemotaxis signal transduction pathway. Nevertheless, several examples have been identified in which dedicated photoreceptor proteins do play a role. The intrinsic complexity of some of these signal transduction systems is overwhelming, in part because of the significant apparent redundancy. The genomics information that is now available for several model organisms (in particular Rhodobacter sphaeroides and Synechocystis sp. PCC6803) facilitates obtaining an increasingly detailed view of the molecular basis of the partial reactions that jointly form the basis of this type of elementary behavioral response.

Tactic responses to oxygen in the phototrophic bacterium Rhodobacter sphaeroides WS8N

The temporal and spatial behavior of a number of mutants of the photosynthetic, facultative anaerobe Rhodobacter sphaeroides to both step changes and to gradients of oxygen was analyzed. Wild-type cells, grown under a range of conditions, showed microaerophilic behavior, accumulating in a 1.3-mm band about 1.3 mm from the meniscus of capillaries. Evidence suggests this is the result of two signaling pathways. The strength of any response depended on the growth and incubation conditions. Deletion of either the complete chemosensory operons 1 and 2 plus the response regulator genes cheY(4) and cheY(5) or cheA(2) alone led to the loss of all aerotactic responses, although the cells still swam normally. The Prr system of R. sphaeroides responds to electron flow through the alternative high-affinity cytochrome oxidase, cbb(3), controlling expression of a wide range of metabolic pathways. Mutants with deletions of either the complete Prr operon or the histidine kinase, PrrB, accumulated up to the meniscus but still formed a thick band 1.3 mm from the aerobic interface. This indicates that the negative aerotactic response to high oxygen levels depends on PrrB, but the mutant cells still retain the positive response. Tethered PrrB(-) cells also showed no response to a step-down in oxygen concentration, although those with deletions of the whole operon showed some response. In gradients of oxygen where the concentration was reduced at 0.4 micro M/s, tethered wild-type cells showed two different phases of response, with an increase in stopping frequency when the oxygen concentration fell from 80 to 50% dissolved oxygen and a decrease in stopping at 50 to 20% dissolved oxygen, with cells returning to their normal stopping frequency in 0% oxygen. PrrB and CheA(2) mutants showed no response, while PrrCBA mutants still showed some response.

Component of the Rhodospirillum centenum photosensory apparatus with structural and functional similarity to methyl-accepting chemotaxis protein chemoreceptors

Photosynthetic bacteria respond to alterations in light conditions by migrating to locations that allows optimal use of light as an energy source. Studies have indicated that photosynthesis-driven electron transport functions as an attractant signal for motility among purple photosynthetic bacteria. However, it is unclear just how the motility-based signal transduction system monitors electron flow through photosynthesis-driven electron transport. Recently, we have demonstrated that the purple photosynthetic bacterium Rhodospirillum centenum is capable of rapidly moving swarm cell colonies toward infrared light as well as away from visible light. Light-driven colony motility of R. centenum has allowed us to perform genetic dissection of the signaling pathway that affects photosynthesis-driven motility. In this study, we have undertaken sequence and mutational analyses of one of the components of a signal transduction pathway, Ptr, which appears responsible for transmitting a signal from the photosynthesis-driven electron transport chain to the chemotaxis signal transduction cascade. Mutational analysis demonstrates that cells disrupted for ptr are defective in altering motility in response to light, as well as defective in light-dependent release of methanol. We present a model which proposes that Ptr senses the redox state of a component in the photosynthetic cyclic electron transport chain and that Ptr is responsible for transmitting a signal to the chemotaxis machinery to induce a photosynthesis-dependent motility response.

Behavioral responses of Rhodobacter sphaeroides to linear gradients of the nutrients succinate and acetate

Rhodobacter sphaeroides cells were tethered by their flagella and subjected to increasing and decreasing nutrient gradients. Using motion analysis, changes in flagellar motor rotation were measured and the responses of the cells to the chemotactic gradients were determined. The steepness and concentration ranges of increasing and decreasing gradients were varied, and the bacterial responses were measured. This allowed the limits of gradients that would invoke changes in flagellar behavior to be determined and thus predicts the nature of gradients that would evoke chemotaxis in the environment. The sensory threshold was measured at 30 nM, and the response showed saturation at 150 microM. The study determined that cells detected and responded to changing concentration rates as low as 1 nM/s for acetate and 5 nM/s for succinate. The complex sensory system of R. sphaeroides responded to both increasing and decreasing concentration gradients of attractant with different sensitivities. In addition, transition phases involving changes in the motor speed and the smoothness of motor rotation were found.

Color-sensitive motility and methanol release responses in Rhodobacter sphaeroides

Blue-light-induced repellent and demethylation responses, characteristic of behavioral adaptation, were observed in Rhodobacter sphaeroides. They were analyzed by computer-assisted motion analysis and through the release of volatile tritiated compounds from [methyl-(3)H]methionine-labeled cells, respectively. Increases in the stop frequency and the rate of methanol release were induced by exposure of cells to repellent light signals, such as an increase in blue- and a decrease in infrared-light intensity. At a lambda of >500 nm the amplitude of the methanol release response followed the absorbance spectrum of the photosynthetic pigments, suggesting that they function as photosensors for this response. In contrast to the previously reported motility response to a decrease in infrared light, the blue-light response reported here does not depend on the number of photosynthetic pigments per cell, suggesting that it is mediated by a separate sensor. Therefore, color discrimination in taxis responses in R. sphaeroides involves two photosensing systems: the photosynthetic pigments and an additional photosensor, responding to blue light. The signal generated by the former system could result in the migration of cells to a light climate beneficial for photosynthesis, while the blue-light system could allow cells to avoid too-high intensities of (harmful) blue light.

Response kinetics of tethered Rhodobacter sphaeroides to changes in light intensity

Rhodobacter sphaeroides can swim toward a wide range of attractants (a process known as taxis), propelled by a single rotating flagellum. The reversals of motor direction that cause tumbles in Eschericia coli taxis are replaced by brief motor stops, and taxis is controlled by a complex sensory system with multiple homologues of the E. coli sensory proteins. We tethered photosynthetically grown cells of R. sphaeroides by their flagella and measured the response of the flagellar motor to changes in light intensity. The unstimulated bias (probability of not being stopped) was significantly larger than the bias of tethered E. coli but similar to the probability of not tumbling in swimming E. coli. Otherwise, the step and impulse responses were the same as those of tethered E. coli to chemical attractants. This indicates that the single motor and multiple sensory signaling pathways in R. sphaeroides generate the same swimming response as several motors and a single pathway in E. coli, and that the response of the single motor is directly observable in the swimming pattern. Photo-responses were larger in the presence of cyanide or the uncoupler carbonyl cyanide 4-trifluoromethoxyphenylhydrazone (FCCP), consistent with the photo-response being detected via changes in the rate of electron transport.

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 tactic responses

Many, if not most, bacterial species swim. The synthesis and operation of the flagellum, the most complex organelle of a bacterium, takes a significant percentage of cellular energy, particularly in the nutrient limited environments in which many motile species are found. It is obvious that motility accords cells a survival advantage over non-motile mutants under normal, poorly mixed conditions and is an important determinant in the development of many associations between bacteria and other organisms, whether as pathogens or symbionts and in colonization of niches and the development of biofilms. This survival advantage is the result of sensory control of swimming behaviour. Although too small to sense a gradient along the length of the cell, and unable to swim great distances because of buffetting by Brownian motion and the curvature resulting from a rotating flagellum, bacteria can bias their random swimming direction towards a more favourable environment. The favourable environment will vary from species to species and there is now evidence that in many species this can change depending on the current physiological growth state of the cell. In general, bacteria sense changes in a range of nutrients and toxins, compounds altering electron transport, acceptors or donors into the electron transport chain, pH, temperature and even the magnetic field of the Earth. The sensory signals are balanced, and may be balanced with other sensory pathways such as quorum sensing, to identify the optimum current environment. The central sensory pathway in this process is common to most bacteria and most effectors. The environmental change is sensed by a sensory protein. In most species examined this is a transmembrane protein, sensing the external environment, but there is increasing evidence for additional cytoplasmic receptors in many species. All receptors, whether sensing sugars, amino acids or oxygen, share a cytoplasmic signalling domain that controls the activity of a histidine protein kinase, CheA, via a linker protein, CheW. A reduction in an attractant generally leads to the increased autophosphorylation of CheA. CheA passes its phosphate to a small, single domain response regulator, CheY. CheY-P can interact with the flagellar motor to cause it to change rotational direction or stop. Signal termination either via a protein, CheZ, which increases the dephosphorylation rate of CheY-P or via a second CheY which acts as a phosphate sink, allows the cell to swim off again, usually in a new direction. In addition to signal termination the receptor must be reset, and this occurs via methylation of the receptor to return it to a non-signalling conformation. The way in which bacteria use these systems to move to optimum environments and the interaction of the different sensory pathways to produce species-specific behavioural response will be the subject of this review.