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Growth and development: prokaryotes
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Growth and development: prokaryotes Editorial overview Orna Amster-Choder and Taˆm Mignot Current Opinion in Microbiology 2012, 15:705–706 For a complete overview see the Issue 1369-5274/$ – see front matter, # 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mib.2012.11.003
Orna Amster-Choder Department of Microbiology and Molecular Genetics, Hebrew University of Jerusalem, Jerusalem, Israel e-mail: [email protected] Orna Amster-Choder is a professor in the Faculty of Medicine at the Hebrew University in Jerusalem, where she holds the Dr Jacob Grunbaum Professional Chair in Medical Sciences. Research in her group focuses on molecular mechanisms underlying signal transduction in bacteria, spatial and temporal organization of proteins and RNA transcripts in bacterial cells, transcription regulation, and molecular mechanisms underlying bacterial pathogenicity. e-mail: [email protected].
Taˆm Mignot Laboratoire de Chimie Bacte´rienne, CNRSAix Marseille University, 31, chemin Joseph Aiguier, Marseille, France e-mail: [email protected] Taˆm Mignot is a CNRS group leader at the Laboratoire de Chimie Bacte´rienne, a division of the Institut de Microbiologie de la Me´diterrane´e (CNRS-Aix Marseille University). Research in his group focuses on the molecular and cellular mechanisms of bacterial surface motility using Myxococcus xanthus as a model system. Interests of the group include the mechanics of motility, its spatial regulation, signal transduction, chemotaxis, cell–cell signaling and the evolution of bacterial cooperative behaviors.
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The existence of bacterial subcellular organelles has been known since the 1960s, but these structures were considered exotic examples and peculiarities of a restricted number of bacteria: magnetotactic bacteria, photosynthetic bacteria, carbon-fixing bacteria, and so on. Despite these examples, the bacterial cell has long been viewed as noncompartmentalized, with most biochemical reactions occurring by random collisions, regulated by diffusion and concentration. This concept was first shaken in the early 1990s with the discovery of polar chemoreceptors in Escherichia coli, indicating for the first time that biochemical processes may also undergo spatial regulations. Because of the improvement in fluorescence microscopy and the development of sophisticated fluorescence probes, the exception became the norm, leading to a new image of the bacterial cell as highly organized. In the ‘new’ bacterial cell, growth is supported by a complex cytoskeletal network of tubulin-like proteins, actin-like proteins and intermediate filament-like proteins. The chromosome is organized in spatial domains, for replication, segregation and gene expression. All cellular processes show some level of spatial regulation: metabolic processing, signal transduction, translation (separately from transcription) and assembly of large macromolecular machineries. The overarching theme to the reviews presented in this issue is how spatial information translates into phenotype, discussing both the origins and the role of localization. The integrity and shape of the bacterial cell is maintained by the peptidoglycan (PG). Since PG forms a single macromolecule around the bacterial membrane, it must be continuously remodeled to accommodate growth and the insertion of protein complexes into the cell envelope. While much knowledge has accumulated on PG biosynthetic pathways, how PG synthesis is coordinated in space and time remained poorly understood. On the basis of recent imaging studies, Huang et al. propose, rather thoughtprovokingly, that PG growth follows a chiral path due to the underlying connection with the helical MreB cytoskeleton. In this system, chirality would emerge from the rotation of PG synthetic complexes guided by short right-handed MreB helical filaments. While further experimental analyses may be needed to corroborate this idea, it suggests that underlying PG helicity is a fundamental spatial pattern of the bacterial cell. Thus, the topology of the glycan strands in the PG meshwork may directly impact the localization of surface proteins. In support of this view, Bierne and Dramsi show that several Gram-positive virulence proteins that are covalently anchored to the PG are in fact arranged helically along the cell axis. Importantly, changes in environmental conditions may cause shifts in the activity of the cell wall synthetic and/or turnover machineries, leading to polar accumulation of these virulence proteins. These changes can affect pathogenesis directly: in the case of ActA, a gradual shift to polar localization allows actin-based motility in the cytosol of Listeria host cells. Current Opinion in Microbiology 2012, 15:705–706
706 Growth and development: prokaryotes
The bacterial membrane may also contain positional information. Fishov and Norris discuss the existence of lipid microdomains in the bacterial membrane. Specific lipids may thus be used to target proteins to the bacterial pole, cluster signal transduction proteins and coordinate protein secretion. How is lipid heterogeneity established? Specific lipid geometries may target them to subcellular sites (i.e. accumulation of cardiolipids in strongly curved regions) and lipid-binding proteins may act as organizers of lipid rafts. Moreover, Fishov and Norris propose that coupling of translation to the insertion of integral membrane proteins may be a major generator of membrane heterogeneity. In this model, the insertion of membrane protein with specific lipid preference would attract these lipids to membrane subdomains. Importantly, the model also predicts that lipid domains organize the bacterial nucleoid in a feedback mechanism. In addition to lipid composition, membrane geometry may also dictate protein localization. Strahl and Hamoen discuss the preferential localization of the Bacillus subtilis DivIVA protein to concave regions at the cell poles and the nascent septum. Interestingly, this binding preference may be explained by a few properties of DivIVA: weak interactions between DivIVA subunits and the membrane may be favored by a concave membrane geometry. If correct, this hypothesis suggests that protein targeting to negative membranes may not be an idiosyncratic property of DivIVA and may be broadly used to target proteins to the bacterial poles. Why is it important to target protein to subcellular sites and why specifically to the cell pole? In B. subtilis, polar DivIVA controls division site placement and anchors the chromosome at the cell pole during sporulation. In Streptomyces coelicolor, DivIVA controls growth of the aerial hyphaes, formed by extended filamentous cells with multiple branches. In their review, Fla¨rdh et al. explain that DivIVA localizes at the tip of a growing Streptomyces filament and likely promotes growth by recruiting the PG synthetic machinery to this site. This fascinating mechanism also allows branching because polar DivIVA clusters split occasionally and thus create new PG-synthesis sites along the growing filament. Branching is further controlled by the phosphorylation of DivIVA at the tip. Thus, in Streptomyces, polar localization of DivIVA and its spatial regulation translate directly into morphogenesis. In Caulobacter crescentus, polar protein localization determines cell fate. Caulobacter cells divide asymmetrically to produce two distinct daughter cells, the swarmer cell and the stalked cell. Only the stalked cell is proficient for DNA replication. Tsokos and Laub discuss the mechanism of replicative asymmetry and suggest that it results
Current Opinion in Microbiology 2012, 15:705–706
from differential allosteric regulations of the CckA kinase at opposite cell poles. At the swarmer cell pole, CckA is active and phosphorylates the CtrA response regulator to block the replication origin. In contrast, at the stalked cell pole, CckA is converted into a phosphatase and, because unphosphoryated CtrA is prone to degradation, the replication origin is accessible. Importantly, this regulation occurs in predivisional cells before cytokinesis, suggesting that differential spatial localization of CckA creates cellular microenvironments despite the absence of a physical barrier. In the last example, Kaimer et al. show that regulated cell polarity can drive bacterial morphogenetic movements. Myxococcus xanthus cells move collectively to build multicellular fruiting bodies under starvation conditions. This cooperative behavior requires the coordination of two motility engines at one pole, which is controlled by a Ras-like small G-protein, MglA. Remarkably, upon activation, a signal transduction cascade (Frz) provokes the relocalization of MglA to the opposite cell pole, allowing a rapid change in the direction of movement. Thus, in Myxococcus dynamic regulators define a cellular compass, much like in chemotaxing eukaryotic cells. In this issue of Current Opinion in Microbiology, selected examples illustrate the importance of spatial regulations in bacteria. A myriad of other examples have accumulated since the early 2000s involving spatial regulations in virtually all cellular processes. However, understanding the mechanisms and function of localization remain a major challenge. From a pure microscopy perspective, the size of the fluorescence objects is highly limiting, since the size of the bacterial cell approaches the diffraction limit of light. A promising research avenue resides in computational microscopy techniques, otherwise called single-molecule superresolution microscopy, which overcome this diffraction limit. Cattoni et al. describe recent developments in this exciting field of research. These methodologies (PALM/STORM) capture the fluorescence emission of a single photoactivable probe and map its precise localization by its point spread function. This way, a localization map of all single emitters produces a high-resolution 2D image of the fluorescent object. In bacteria, PALM has proven promising to resolve stochastic chemoreceptor clustering and STORM was applied to survey the distribution of nucleoid-associated proteins on the bacterial chromosome; problems that could not be solved by standard light microscopy. These fast developing technologies are being adapted to 3D imaging and live time lapse studies, which will likely bring bacterial cell biology to new frontiers.
www.sciencedirect.com
Growth and development: prokaryotes Editorial overview Orna Amster-Choder and Taˆm Mignot Current Opinion in Microbiology 2012, 15:705–706 For a complete overview see the Issue 1369-5274/$ – see front matter, # 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mib.2012.11.003
Orna Amster-Choder Department of Microbiology and Molecular Genetics, Hebrew University of Jerusalem, Jerusalem, Israel e-mail: [email protected] Orna Amster-Choder is a professor in the Faculty of Medicine at the Hebrew University in Jerusalem, where she holds the Dr Jacob Grunbaum Professional Chair in Medical Sciences. Research in her group focuses on molecular mechanisms underlying signal transduction in bacteria, spatial and temporal organization of proteins and RNA transcripts in bacterial cells, transcription regulation, and molecular mechanisms underlying bacterial pathogenicity. e-mail: [email protected].
Taˆm Mignot Laboratoire de Chimie Bacte´rienne, CNRSAix Marseille University, 31, chemin Joseph Aiguier, Marseille, France e-mail: [email protected] Taˆm Mignot is a CNRS group leader at the Laboratoire de Chimie Bacte´rienne, a division of the Institut de Microbiologie de la Me´diterrane´e (CNRS-Aix Marseille University). Research in his group focuses on the molecular and cellular mechanisms of bacterial surface motility using Myxococcus xanthus as a model system. Interests of the group include the mechanics of motility, its spatial regulation, signal transduction, chemotaxis, cell–cell signaling and the evolution of bacterial cooperative behaviors.
www.sciencedirect.com
The existence of bacterial subcellular organelles has been known since the 1960s, but these structures were considered exotic examples and peculiarities of a restricted number of bacteria: magnetotactic bacteria, photosynthetic bacteria, carbon-fixing bacteria, and so on. Despite these examples, the bacterial cell has long been viewed as noncompartmentalized, with most biochemical reactions occurring by random collisions, regulated by diffusion and concentration. This concept was first shaken in the early 1990s with the discovery of polar chemoreceptors in Escherichia coli, indicating for the first time that biochemical processes may also undergo spatial regulations. Because of the improvement in fluorescence microscopy and the development of sophisticated fluorescence probes, the exception became the norm, leading to a new image of the bacterial cell as highly organized. In the ‘new’ bacterial cell, growth is supported by a complex cytoskeletal network of tubulin-like proteins, actin-like proteins and intermediate filament-like proteins. The chromosome is organized in spatial domains, for replication, segregation and gene expression. All cellular processes show some level of spatial regulation: metabolic processing, signal transduction, translation (separately from transcription) and assembly of large macromolecular machineries. The overarching theme to the reviews presented in this issue is how spatial information translates into phenotype, discussing both the origins and the role of localization. The integrity and shape of the bacterial cell is maintained by the peptidoglycan (PG). Since PG forms a single macromolecule around the bacterial membrane, it must be continuously remodeled to accommodate growth and the insertion of protein complexes into the cell envelope. While much knowledge has accumulated on PG biosynthetic pathways, how PG synthesis is coordinated in space and time remained poorly understood. On the basis of recent imaging studies, Huang et al. propose, rather thoughtprovokingly, that PG growth follows a chiral path due to the underlying connection with the helical MreB cytoskeleton. In this system, chirality would emerge from the rotation of PG synthetic complexes guided by short right-handed MreB helical filaments. While further experimental analyses may be needed to corroborate this idea, it suggests that underlying PG helicity is a fundamental spatial pattern of the bacterial cell. Thus, the topology of the glycan strands in the PG meshwork may directly impact the localization of surface proteins. In support of this view, Bierne and Dramsi show that several Gram-positive virulence proteins that are covalently anchored to the PG are in fact arranged helically along the cell axis. Importantly, changes in environmental conditions may cause shifts in the activity of the cell wall synthetic and/or turnover machineries, leading to polar accumulation of these virulence proteins. These changes can affect pathogenesis directly: in the case of ActA, a gradual shift to polar localization allows actin-based motility in the cytosol of Listeria host cells. Current Opinion in Microbiology 2012, 15:705–706
706 Growth and development: prokaryotes
The bacterial membrane may also contain positional information. Fishov and Norris discuss the existence of lipid microdomains in the bacterial membrane. Specific lipids may thus be used to target proteins to the bacterial pole, cluster signal transduction proteins and coordinate protein secretion. How is lipid heterogeneity established? Specific lipid geometries may target them to subcellular sites (i.e. accumulation of cardiolipids in strongly curved regions) and lipid-binding proteins may act as organizers of lipid rafts. Moreover, Fishov and Norris propose that coupling of translation to the insertion of integral membrane proteins may be a major generator of membrane heterogeneity. In this model, the insertion of membrane protein with specific lipid preference would attract these lipids to membrane subdomains. Importantly, the model also predicts that lipid domains organize the bacterial nucleoid in a feedback mechanism. In addition to lipid composition, membrane geometry may also dictate protein localization. Strahl and Hamoen discuss the preferential localization of the Bacillus subtilis DivIVA protein to concave regions at the cell poles and the nascent septum. Interestingly, this binding preference may be explained by a few properties of DivIVA: weak interactions between DivIVA subunits and the membrane may be favored by a concave membrane geometry. If correct, this hypothesis suggests that protein targeting to negative membranes may not be an idiosyncratic property of DivIVA and may be broadly used to target proteins to the bacterial poles. Why is it important to target protein to subcellular sites and why specifically to the cell pole? In B. subtilis, polar DivIVA controls division site placement and anchors the chromosome at the cell pole during sporulation. In Streptomyces coelicolor, DivIVA controls growth of the aerial hyphaes, formed by extended filamentous cells with multiple branches. In their review, Fla¨rdh et al. explain that DivIVA localizes at the tip of a growing Streptomyces filament and likely promotes growth by recruiting the PG synthetic machinery to this site. This fascinating mechanism also allows branching because polar DivIVA clusters split occasionally and thus create new PG-synthesis sites along the growing filament. Branching is further controlled by the phosphorylation of DivIVA at the tip. Thus, in Streptomyces, polar localization of DivIVA and its spatial regulation translate directly into morphogenesis. In Caulobacter crescentus, polar protein localization determines cell fate. Caulobacter cells divide asymmetrically to produce two distinct daughter cells, the swarmer cell and the stalked cell. Only the stalked cell is proficient for DNA replication. Tsokos and Laub discuss the mechanism of replicative asymmetry and suggest that it results
Current Opinion in Microbiology 2012, 15:705–706
from differential allosteric regulations of the CckA kinase at opposite cell poles. At the swarmer cell pole, CckA is active and phosphorylates the CtrA response regulator to block the replication origin. In contrast, at the stalked cell pole, CckA is converted into a phosphatase and, because unphosphoryated CtrA is prone to degradation, the replication origin is accessible. Importantly, this regulation occurs in predivisional cells before cytokinesis, suggesting that differential spatial localization of CckA creates cellular microenvironments despite the absence of a physical barrier. In the last example, Kaimer et al. show that regulated cell polarity can drive bacterial morphogenetic movements. Myxococcus xanthus cells move collectively to build multicellular fruiting bodies under starvation conditions. This cooperative behavior requires the coordination of two motility engines at one pole, which is controlled by a Ras-like small G-protein, MglA. Remarkably, upon activation, a signal transduction cascade (Frz) provokes the relocalization of MglA to the opposite cell pole, allowing a rapid change in the direction of movement. Thus, in Myxococcus dynamic regulators define a cellular compass, much like in chemotaxing eukaryotic cells. In this issue of Current Opinion in Microbiology, selected examples illustrate the importance of spatial regulations in bacteria. A myriad of other examples have accumulated since the early 2000s involving spatial regulations in virtually all cellular processes. However, understanding the mechanisms and function of localization remain a major challenge. From a pure microscopy perspective, the size of the fluorescence objects is highly limiting, since the size of the bacterial cell approaches the diffraction limit of light. A promising research avenue resides in computational microscopy techniques, otherwise called single-molecule superresolution microscopy, which overcome this diffraction limit. Cattoni et al. describe recent developments in this exciting field of research. These methodologies (PALM/STORM) capture the fluorescence emission of a single photoactivable probe and map its precise localization by its point spread function. This way, a localization map of all single emitters produces a high-resolution 2D image of the fluorescent object. In bacteria, PALM has proven promising to resolve stochastic chemoreceptor clustering and STORM was applied to survey the distribution of nucleoid-associated proteins on the bacterial chromosome; problems that could not be solved by standard light microscopy. These fast developing technologies are being adapted to 3D imaging and live time lapse studies, which will likely bring bacterial cell biology to new frontiers.
www.sciencedirect.com