Divergent regulatory pathways control A and S motility in Myxococcus xanthus through FrzE, a CheA-CheY fusion protein

Regulation of Cell Polarity in Motility and Cell Division in Myxococcus xanthus

Rod-shaped Myxococcus xanthus cells are polarized with proteins asymmetrically localizing to specific positions. This spatial organization is important for regulation of motility and cell division and changes over time. Dedicated protein modules regulate motility independent of the cell cycle, and cell division dependent on the cell cycle. For motility, a leading-lagging cell polarity is established that is inverted during cellular reversals. Establishment and inversion of this polarity are regulated hierarchically by interfacing protein modules that sort polarized motility proteins to the correct cell poles or cause their relocation between cell poles during reversals akin to a spatial toggle switch. For division, a novel self-organizing protein module that incorporates a ParA ATPase positions the FtsZ-ring at midcell. This review covers recent findings concerning the spatiotemporal regulation of motility and cell division in M. xanthus and illustrates how the study of diverse bacteria may uncover novel mechanisms involved in regulating bacterial cell polarity.

Pervasive, yet idiosyncratic, epistatic pleiotropy during adaptation in a behaviourally complex microbe

Understanding how multiple mutations interact to jointly impact multiple ecologically important traits is critical for creating a robust picture of organismal fitness and the process of adaptation. However, this is complicated by both environmental heterogeneity and the complexity of genotype-to-phenotype relationships generated by pleiotropy and epistasis. Moreover, little is known about how pleiotropic and epistatic relationships themselves change over evolutionary time. The soil bacterium Myxococcus xanthus employs several distinct social traits across a range of environments. Here, we use an experimental lineage of M. xanthus that evolved a novel form of social motility to address how interactions between epistasis and pleiotropy evolve. Specifically, we test how mutations accumulated during selection on soft agar pleiotropically affect several other social traits (hard agar motility, predation and spore production). Relationships between changes in swarming rate in the selective environment and the four other traits varied greatly over time in both direction and magnitude, both across timescales of the entire evolutionary lineage and individual evolutionary time steps. We also tested how a previously defined epistatic interaction is pleiotropically expressed across these traits. We found that phenotypic effects of this epistatic interaction were highly correlated between soft and hard agar motility, but were uncorrelated between soft agar motility and predation, and inversely correlated between soft agar motility and spore production. Our results show that 'epistatic pleiotropy' varied greatly in magnitude, and often even in sign, across traits and over time, highlighting the necessity of simultaneously considering the interacting complexities of pleiotropy and epistasis when studying the process of adaptation.

Directional reversals enable Myxococcus xanthus cells to produce collective one-dimensional streams during fruiting-body formation

The formation of a collectively moving group benefits individuals within a population in a variety of ways. The surface-dwelling bacterium Myxococcus xanthus forms dynamic collective groups both to feed on prey and to aggregate during times of starvation. The latter behaviour, termed fruiting-body formation, involves a complex, coordinated series of density changes that ultimately lead to three-dimensional aggregates comprising hundreds of thousands of cells and spores. How a loose, two-dimensional sheet of motile cells produces a fixed aggregate has remained a mystery as current models of aggregation are either inconsistent with experimental data or ultimately predict unstable structures that do not remain fixed in space. Here, we use high-resolution microscopy and computer vision software to spatio-temporally track the motion of thousands of individuals during the initial stages of fruiting-body formation. We find that cells undergo a phase transition from exploratory flocking, in which unstable cell groups move rapidly and coherently over long distances, to a reversal-mediated localization into one-dimensional growing streams that are inherently stable in space. These observations identify a new phase of active collective behaviour and answer a long-standing open question in Myxococcus development by describing how motile cell groups can remain statistically fixed in a spatial location.

Phosphorylation-dependent localization of the response regulator FrzZ signals cell reversals in Myxococcus xanthus

The life cycle of Myxococcus xanthus includes co-ordinated group movement and fruiting body formation, and requires directed motility and controlled cell reversals. Reversals are achieved by inverting cell polarity and re-organizing many motility proteins. The Frz chemosensory pathway regulates the frequency of cell reversals. While it has been established that phosphotransfer from the kinase FrzE to the response regulator FrzZ is required, it is unknown how phosphorylated FrzZ, the putative output of the pathway, targets the cell polarity axis. In this study, we used Phos-tag SDS-PAGE to determine the cellular level of phospho-FrzZ under different growth conditions and in Frz signalling mutants. We detected consistent FrzZ phosphorylation, albeit with a short half-life, in cells grown on plates, but not from liquid culture. The available pool of phospho-FrzZ correlated with reversal frequencies, with higher levels found in hyper-reversing mutants. Phosphorylation was not detected in hypo-reversing mutants. Fluorescence microscopy revealed that FrzZ is recruited to the leading cell pole upon phosphorylation and switches to the opposite pole during reversals. These results are consistent with the hypothesis that the Frz pathway modulates reversal frequency through a localized response regulator that targets cell polarity regulators at the leading cell pole.

Genetic analysis of the regulation of type IV pilus function by the Chp chemosensory system of Pseudomonas aeruginosa

The virulence of the opportunistic pathogen Pseudomonas aeruginosa involves the coordinate expression of many virulence factors, including type IV pili, which are required for colonization of host tissues and for twitching motility. Type IV pilus function is controlled in part by the Chp chemosensory system, which includes a histidine kinase, ChpA, and two CheY-like response regulators, PilG and PilH. How the Chp components interface with the type IV pilus motor proteins PilB, PilT, and PilU is unknown. We present genetic evidence confirming the role of ChpA, PilG, and PilB in the regulation of pilus extension and the role of PilH and PilT in regulating pilus retraction. Using informative double and triple mutants, we show that (i) ChpA, PilG, and PilB function upstream of PilH, PilT, and PilU; (ii) that PilH enhances PilT function; and (iii) that PilT and PilB retain some activity in the absence of signaling input from components of the Chp system. By site-directed mutagenesis, we demonstrate that the histidine kinase domain of ChpA and the phosphoacceptor sites of both PilG and PilH are required for type IV pilus function, suggesting that they form a phosphorelay system important in the regulation of pilus extension and retraction. Finally, we present evidence suggesting that pilA transcription is regulated by intracellular PilA levels. We show that PilA is a negative regulator of pilA transcription in P. aeruginosa and that the Chp system functionally regulates pilA transcription by controlling PilA import and export.

Comparative genomics of Geobacter chemotaxis genes reveals diverse signaling function

Background

Geobacter species are δ-Proteobacteria and are often the predominant species in a variety of sedimentary environments where Fe(III) reduction is important. Their ability to remediate contaminated environments and produce electricity makes them attractive for further study. Cell motility, biofilm formation, and type IV pili all appear important for the growth of Geobacter in changing environments and for electricity production. Recent studies in other bacteria have demonstrated that signaling pathways homologous to the paradigm established for Escherichia coli chemotaxis can regulate type IV pili-dependent motility, the synthesis of flagella and type IV pili, the production of extracellular matrix material, and biofilm formation. The classification of these pathways by comparative genomics improves the ability to understand how Geobacter thrives in natural environments and better their use in microbial fuel cells.

Results

The genomes of G. sulfurreducens, G. metallireducens, and G. uraniireducens contain multiple (~70) homologs of chemotaxis genes arranged in several major clusters (six, seven, and seven, respectively). Unlike the single gene cluster of E. coli, the Geobacter clusters are not all located near the flagellar genes. The probable functions of some Geobacter clusters are assignable by homology to known pathways; others appear to be unique to the Geobacter sp. and contain genes of unknown function. We identified large numbers of methyl-accepting chemotaxis protein (MCP) homologs that have diverse sensing domain architectures and generate a potential for sensing a great variety of environmental signals. We discuss mechanisms for class-specific segregation of the MCPs in the cell membrane, which serve to maintain pathway specificity and diminish crosstalk. Finally, the regulation of gene expression in Geobacter differs from E. coli. The sequences of predicted promoter elements suggest that the alternative sigma factors σ²⁸ and σ⁵⁴ play a role in regulating the Geobacter chemotaxis gene expression.

Conclusion

The numerous chemoreceptors and chemotaxis-like gene clusters of Geobacter appear to be responsible for a diverse set of signaling functions in addition to chemotaxis, including gene regulation and biofilm formation, through functionally and spatially distinct signaling pathways.

Genetic circuitry controlling motility behaviors of Myxococcus xanthus

M. xanthus has a complex multicellular lifestyle including swarming, predation and development. These behaviors depend on the ability of the cells to achieve directed motility across solid surfaces. M. xanthus cells have evolved two motility systems including Type-IV pili that act as grappling hooks and a controversial engine involving mucus secretion and fixed focal adhesion sites. The necessity for cells to coordinate the motility systems and to respond rapidly to environmental cues is reflected by a complex genetic network involving at least three complete sets of chemosensory systems and eukaryotic-like signaling proteins. In this review, we discuss recent advances suggesting that motor synchronization results from spatial oscillations of motility proteins. We further propose that these dynamics are modulated by the action of multiple upstream complementary signaling systems. M. xanthus is thus an exciting emerging model system to study the intricate processes of directed cell migration.

The receiver domain of FrzE, a CheA-CheY fusion protein, regulates the CheA histidine kinase activity and downstream signalling to the A- and S-motility systems of Myxococcus xanthus

The Frz chemosensory system is a two-component signal transduction pathway that controls cell reversals and directional movements for the two motility systems in Myxococcus xanthus. To trigger cell reversals, FrzE, a hybrid CheA-CheY fusion protein, autophosphorylates the kinase domain at His-49, and phosphoryl groups are transferred to aspartate residues (Asp-52 and Asp-220) in the two receiver domains of FrzZ, a dual CheY-like protein that serves as the pathway output. The role of the receiver domain of FrzE was unknown. In this paper, we characterize the FrzE protein in vitro and show that the receiver domain of FrzE negatively regulates the autophosphorylation activity of the kinase domain of FrzE. Unexpectedly, it does not appear to play a direct role in phospho-relay as in most other histidine kinase receiver domain hybrid systems. The regulatory role of the FrzE receiver domain suggests that it may interact with or be phosphorylated by an unknown protein. We also show the dynamics of motility system-specific marker proteins in FrzE mutants as cells move forward and reverse. Our studies indicate that the two motility systems are functionally co-ordinated and that any system-specific branching of the pathway most likely occurs downstream of FrzE.

FrzZ, a dual CheY-like response regulator, functions as an output for the Frz chemosensory pathway of Myxococcus xanthus

Myxococcus xanthus utilizes two distinct motility systems for movement (gliding) on solid surfaces: adventurous motility (A-motility) and social motility (S-motility). Both systems are regulated by the Frz signal transduction pathway, which controls cell reversals required for directed motility and fruiting body formation. The Frz chemosensory system, unlike the Escherichia coli chemotaxis system, contains proteins with multiple response regulator domains: FrzE, a CheA-CheY hybrid protein, and FrzZ, a CheY-CheY hybrid protein. Previously, the CheY domain of FrzE was hypothesized to act as the response regulator output of the Frz system. In this study, using a genetic suppressor screen, we identified FrzZ and showed FrzZ is epistatic to FrzE, demonstrating that FrzZ is the principal output component of the pathway. We constructed M. xanthus point mutations in the phosphoaccepting aspartate residues of FrzZ and demonstrated the respective roles of these residues in group and single cell motility. We also performed in vitro assays and showed rapid phosphotransfer between the CheA domain of FrzE and each of the CheY domains of FrzZ. These experiments showed that FrzZ plays a direct role as an output of the Frz chemosensory pathway and that both CheY domains of FrzZ are functional.

Beta-D-Allose inhibits fruiting body formation and sporulation in Myxococcus xanthus

Myxococcus xanthus, a gram-negative soil bacterium, responds to amino acid starvation by entering a process of multicellular development which culminates in the assembly of spore-filled fruiting bodies. Previous studies utilizing developmental inhibitors (such as methionine, lysine, or threonine) have revealed important clues about the mechanisms involved in fruiting body formation. We used Biolog phenotype microarrays to screen 384 chemicals for complete inhibition of fruiting body development in M. xanthus. Here, we report the identification of a novel inhibitor of fruiting body formation and sporulation, beta-d-allose. beta-d-Allose, a rare sugar, is a member of the aldohexose family and a C3 epimer of glucose. Our studies show that beta-d-allose does not affect cell growth, viability, agglutination, or motility. However, beta-galactosidase reporters demonstrate that genes activated between 4 and 14 h of development show significantly lower expression levels in the presence of beta-d-allose. Furthermore, inhibition of fruiting body formation occurs only when beta-d-allose is added to submerged cultures before 12 h of development. In competition studies, high concentrations of galactose and xylose antagonize the nonfruiting response to beta-d-allose, while glucose is capable of partial antagonism. Finally, a magellan-4 transposon mutagenesis screen identified glcK, a putative glucokinase gene, required for beta-d-allose-mediated inhibition of fruiting body formation. Subsequent glucokinase activity assays of the glcK mutant further supported the role of this protein in glucose phosphorylation.

Effect of site-specific mutations in different phosphotransfer domains of the chemosensory protein ChpA on Pseudomonas aeruginosa motility

The virulence of Pseudomonas aeruginosa and other surface pathogens involves the coordinate expression of a wide range of virulence determinants, including type IV pili. These surface filaments are important for the colonization of host epithelial tissues and mediate bacterial attachment to, and translocation across, surfaces by a process known as twitching motility. This process is controlled in part by a complex signal transduction system whose central component, ChpA, possesses nine potential sites of phosphorylation, including six histidine-containing phosphotransfer (HPt) domains, one serine-containing phosphotransfer domain, one threonine-containing phosphotransfer domain, and one CheY-like receiver domain. Here, using site-directed mutagenesis, we show that normal twitching motility is entirely dependent on the CheY-like receiver domain and partially dependent on two of the HPt domains. Moreover, under different assay conditions, point mutations in several of the phosphotransfer domains of ChpA give rise to unusual "swarming" phenotypes, possibly reflecting more subtle perturbations in the control of P. aeruginosa motility that are not evident from the conventional twitching stab assay. Together, these results suggest that ChpA plays a central role in the complex regulation of type IV pilus-mediated motility in P. aeruginosa.

Unidirectional movement of flares of cells of Myxococcus xanthus

Amongst other modes, Myxococcal cells move in swarms that are flares or columns of cells. It has been argued that this is a strategy allowing a large enough number of them to encounter food bacteria. Then, the combined large amount of extracellular lytic enzymes from the mass of cells can provide adequate nutrient resources from the food bacteria for all the myxococci of the swarm. However, how they move as a coherent column has not been adequately explained. Here based on the idea that a rare cell can experience a special mutation such that it moves only unidirectionally, a proposal to account for this aspect of Myxococcus cell movement is suggested. Although wild type individual organisms of this species engage in forward and back movements, a mutant cell that moves unidirectionally can bias the movement of associated wild type cells and lead to the formation of a column of cells, headed by such a unique mutated cell. The non-mutated cells follow along it is suggested because of the S-motility (or social motility) system. This may link them to this single unidirectionally moving mutant cell to give a coherent movement to the column. This proposed type of mutation back mutates to wild type and the column no longer functions as such and only wild-type cells are present.

The Dif chemosensory pathway is directly involved in phosphatidylethanolamine sensory transduction inMyxococcus xanthus

Myxococcus xanthus cells glide on solid surfaces and are chemotactically stimulated by certain phosphatidylethanolamine species. The dif gene cluster consists of six genes, difABCDEG, five of which encode proteins homologous to known chemotaxis proteins. DifA and DifE are required for the biosynthesis of fibrils, an extracellular matrix comprised of polysaccharide and protein. Chemotactic stimulation by 1,2-O-Bis[11-(Z)-hexadecenoyl]-sn-glycero-3-phosphatidylethanolamine (16:1 PE) and dilauroyl PE (12:0 PE) requires fibrils. Although previous work has shown that difA and difE mutants are not stimulated by 12:0 PE, these results do not distinguish between a dependence on fibrils or a direct role in chemosensory transduction. Here we provide evidence that the Dif chemosensory pathway directly mediates PE sensory transduction. First, stimulation by and adaptation to 16:1 PE requires all of the dif genes, including difBDG, which are not essential for fibril biogenesis. Second, a specific residue within the first putative methylation domain of DifA is required for stimulation by 16:1 PE but not fibril biogenesis. Transmembrane signalling through a chimeric NarX-DifA chemoreceptor is required for fibril formation but not for stimulation by or adaptation to 16:1 PE. Third, difD and difE are required for stimulation by dioleoyl PE (18:1 PE) although the response does not require fibrils. Taken together these results argue that the Dif pathway mediates both matrix formation and lipid chemotaxis.