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

Emergent Myxobacterial Behaviors Arise from Reversal Suppression Induced by Kin Contacts

A wide range of biological systems, from microbial swarms to bird flocks, display emergent behaviors driven by coordinated movement of individuals. To this end, individual organisms interact by recognizing their kin and adjusting their motility based on others around them. However, even in the best-studied systems, the mechanistic basis of the interplay between kin recognition and motility coordination is not understood. Here, using a combination of experiments and mathematical modeling, we uncover the mechanism of an emergent social behavior in Myxococcus xanthus. By overexpressing the cell surface adhesins TraA and TraB, which are involved in kin recognition, large numbers of cells adhere to one another and form organized macroscopic circular aggregates that spin clockwise or counterclockwise. Mechanistically, TraAB adhesion results in sustained cell-cell contacts that trigger cells to suppress cell reversals, and circular aggregates form as the result of cells’ ability to follow their own cellular slime trails. Furthermore, our in silico simulations demonstrate a remarkable ability to predict self-organization patterns when phenotypically distinct strains are mixed. For example, defying naive expectations, both models and experiments found that strains engineered to overexpress different and incompatible TraAB adhesins nevertheless form mixed circular aggregates. Therefore, this work provides key mechanistic insights into M. xanthus social interactions and demonstrates how local cell contacts induce emergent collective behaviors by millions of cells.

IMPORTANCE In many species, large populations exhibit emergent behaviors whereby all related individuals move in unison. For example, fish in schools can all dart in one direction simultaneously to avoid a predator. Currently, it is impossible to explain how such animals recognize kin through brain cognition and elicit such behaviors at a molecular level. However, microbes also recognize kin and exhibit emergent collective behaviors that are experimentally tractable. Here, using a model social bacterium, we engineer dispersed individuals to organize into synchronized collectives that create emergent patterns. With experimental and mathematical approaches, we explain how this occurs at both molecular and population levels. The results demonstrate how the combination of local physical interactions triggers intracellular signaling, which in turn leads to emergent behaviors on a population scale.

Comparative Genomics of Myxobacterial Chemosensory Systems

Chemosensory systems (CSS) are among the most complex organizations of proteins functioning cooperatively to regulate bacterial motility and other cellular activities. These systems have been studied extensively in bacteria, and usually, they are present as a single system. Eight CSS, the highest number in bacteria, have been reported in Myxococcus xanthus DK1622 and are involved in coordinating diverse functions. Here, we have explored and compared the CSS in all available genomes of order Myxococcales. Myxococcales members contain 97 to 476 two-component system (TCS) proteins, which assist the bacteria in surviving and adapting to varying environmental conditions. The number of myxobacterial CSS ranges between 1 and 12, with the largest number in family Cystobacteraceae and the smallest in Nannocystaceae CheA protein was used as a phylogenetic marker to infer evolutionary relatedness between different CSS, and six novel CSS ("extra CSS" [ECSS]) were thus identified in the myxobacteria besides the previously reported Che1 to Che8 systems from M. xanthus Che1 to Che8 systems are monophyletic to deltaproteobacteria, whereas the newly identified ECSS form separate clades with different bacterial classes. The comparative modular organization was concordant with the phylogeny. Four clusters lacking CheA proteins were also identified via CheB-based phylogenetic analysis and were categorized as accessory CSS (ACSS). In Archangium, an orphan CSS was identified, in which both CheA and CheB were absent. The novel, accessory, and orphan multimodular CSS identified here suggest the emergence of myxobacterial CSS and could assist in further characterizing their roles.IMPORTANCE This study is focused on chemosensory systems (CSS), which help the bacterium in directing its movement toward or away from chemical gradients. CSS are present as a single system in most of the bacteria except in some groups, including Myxococcus xanthus, which has 8 CSS, the highest number reported to date. This is the first comprehensive study carrying out a comparative analysis of the 22 available myxobacterial genomes, which suggests the evolutionary diversity of these systems. We are interested in understanding the distribution of CSS within all known myxobacteria and their probable evolution.

A Clp/Hsp100 chaperone functions in Myxococcus xanthus sporulation and self-organization

The Clp/Hsp100 proteins are chaperones that play a role in protein degradation and reactivation. In bacteria, they exhibit a high degree of pleiotropy, affecting both individual and multicellular phenotypes. In this article, we present the first characterization of a Clp/Hsp100 homolog in Myxococcus xanthus (MXAN_4832 gene locus). Deletion of MXAN_4832 causes defects in both swarming and aggregation related to cell motility and the production of fibrils, which are an important component of the extracellular matrix of a swarm. The deletion also affects the formation of myxospores during development, causing them to become sensitive to heat. The protein product of MXAN_4832 can act as a chaperone in vitro, providing biochemical evidence in support of our hypothesis that MXAN_4832 is a functional Clp/Hsp100 homolog. There are a total of 12 Clp/Hsp100 homologs in M. xanthus, including MXAN_4832, and, based on its mutational and biochemical characterization, they may well represent an important group.

The Myxococcus xanthus spore cuticula protein C is a fragment of FibA, an extracellular metalloprotease produced exclusively in aggregated cells

Myxococcus xanthus is a soil bacterium with a complex life cycle involving distinct cell fates, including production of environmentally resistant spores to withstand periods of nutrient limitation. Spores are surrounded by an apparently self-assembling cuticula containing at least Proteins S and C; the gene encoding Protein C is unknown. During analyses of cell heterogeneity in M. xanthus, we observed that Protein C accumulated exclusively in cells found in aggregates. Using mass spectrometry analysis of Protein C either isolated from spore cuticula or immunoprecipitated from aggregated cells, we demonstrate that Protein C is actually a proteolytic fragment of the previously identified but functionally elusive zinc metalloprotease, FibA. Subpopulation specific FibA accumulation is not due to transcriptional regulation suggesting post-transcriptional regulation mechanisms mediate its heterogeneous accumulation patterns.

DifA, a methyl-accepting chemoreceptor protein-like sensory protein, uses a novel signaling mechanism to regulate exopolysaccharide production in Myxococcus xanthus

DifA is a methyl-accepting chemotaxis protein (MCP)-like sensory transducer that regulates exopolysaccharide (EPS) production in Myxococcus xanthus. Here mutational analysis and molecular biology were used to probe the signaling mechanisms of DifA in EPS regulation. We first identified the start codon of DifA experimentally; this identification extended the N terminus of DifA for 45 amino acids (aa) from the previous bioinformatics prediction. This extension helped to address the outstanding question of how DifA receives input signals from type 4 pili without a prominent periplasmic domain. The results suggest that DifA uses its N-terminus extension to sense an upstream signal in EPS regulation. We suggest that the perception of the input signal by DifA is mediated by protein-protein interactions with upstream components. Subsequent signal transmission likely involves transmembrane signaling instead of direct intramolecular interactions between the input and the output modules in the cytoplasm. The basic functional unit of DifA for signal transduction is likely dimeric as mutational alteration of the predicted dimeric interface of DifA significantly affected EPS production. Deletions of 14-aa segments in the C terminus suggest that the newly defined flexible bundle subdomain in MCPs is likely critical for DifA function because shortening of this bundle can lead to constitutively active mutations.

Phosphorylation and dephosphorylation among Dif chemosensory proteins essential for exopolysaccharide regulation in Myxococcus xanthus

Myxococcus xanthus social gliding motility, which is powered by type IV pili, requires the presence of exopolysaccharides (EPS) on the cell surface. The Dif chemosensory system is essential for the regulation of EPS production. It was demonstrated previously that DifA (methyl-accepting chemotaxis protein [MCP]-like), DifC (CheW-like), and DifE (CheA-like) stimulate whereas DifD (CheY-like) and DifG (CheC-like) inhibit EPS production. DifD was found not to function downstream of DifE in EPS regulation, as a difD difE double mutant phenocopied the difE single mutant. It has been proposed that DifA, DifC, and DifE form a ternary signaling complex that positively regulates EPS production through the kinase activity of DifE. DifD was proposed as a phosphate sink of phosphorylated DifE (DifE approximately P), while DifG would augment the function of DifD as a phosphatase of phosphorylated DifD (DifD approximately P). Here we report in vitro phosphorylation studies with all the Dif chemosensory proteins that were expressed and purified from Escherichia coli. DifE was demonstrated to be an autokinase. Consistent with the formation of a DifA-DifC-DifE complex, DifA and DifC together, but not individually, were found to influence DifE autophosphorylation. DifD, which did not inhibit DifE autophosphorylation directly, was found to accept phosphate from autophosphorylated DifE. While DifD approximately P has an unusually long half-life for dephosphorylation in vitro, DifG efficiently dephosphorylated DifD approximately P as a phosphatase. These results support a model where DifE complexes with DifA and DifC to regulate EPS production through phosphorylation of a downstream target, while DifD and DifG function synergistically to divert phosphates away from DifE approximately P.

Isolation and characterization of a suppressor mutation that restores Myxococcus xanthus exopolysaccharide production

Myxococcus xanthus, a Gram-negative soil bacterium, undergoes multicellular development when nutrients become limiting. Aggregation, which is part of the developmental process, requires the surface motility of this organism. One component of M. xanthus motility, the social (S) gliding motility, enables the movement of cells in close physical proximity. Previous studies demonstrated that the cell surface-associated exopolysaccharide (EPS) is essential for S motility and that the Dif proteins form a chemotaxis-like pathway that regulates EPS production in M. xanthus. DifA, a homologue of methyl-accepting chemotaxis proteins (MCPs) in the Dif system, is required for EPS production, S motility and development. In this study, a spontaneous extragenic suppressor of a difA deletion was isolated in order to identify additional regulators of EPS production. The suppressor mutation was found to be a single base pair insertion in cheW7 at the che7 chemotaxis gene cluster. Further examination indicated that mutations in cheW7 may lead to the interaction of Mcp7 with DifC (CheW-like) and DifE (CheA-like) to reconstruct a functional pathway to regulate EPS production in the absence of DifA. In addition, the cheW7 mutation was found to partially suppress a pilA mutation in EPS production in a difA(+) background. Further deletion of difA from the pilA cheW7 double mutant resulted in a triple mutant that produced wild-type levels of EPS, implying that DifA (MCP-like) and Mcp7 compete for interactions with DifC and DifE in the modulation of EPS production.

Upward mobility and alternative lifestyles: a report from the 10th biennial meeting on Bacterial Locomotion and Signal Transduction

This past January, in Cuernavaca Mexico, a conglomerate of scientists met to discuss the contemporary view of Bacterial Locomotion and Signal Transduction (BLAST). The BLAST meetings represent a field that has its roots in chemotaxis and the flagellum-based motility but now encompass all types of cellular movement and signalling. The topics varied from the interactions between molecules to the interactions between species. We heard about 3D reconstructions of transmembrane chemoreceptors within cells, new biophysical methods for understanding cellular engines, intricate phosphorelays, elaborate gene networks, new messenger molecules and emerging behaviours within complex populations of cells. At BLAST X we gained an appreciation for the lifestyle choices bacteria make, how they get to where they are going and the molecular mechanisms that underlie their decisions. Herein we review the highlights of the meeting.

Deciphering the hunting strategy of a bacterial wolfpack

Myxococcus xanthus is a common soil bacterium with an intricate multicellular lifestyle that continues to challenge the way in which we conceptualize the capabilities of prokaryotic organisms. Myxococcus xanthus is the preferred laboratory representative from the Myxobacteria, a family of organisms distinguished by their ability to form highly structured biofilms that include tentacle-like packs of surface-gliding cell groups, synchronized rippling waves of oscillating cells and massive spore-filled aggregates that protrude upwards from the substratum to form fruiting bodies. But most of the Myxobacteria are also predators that thrive on the degradation of macromolecules released through the lysis of other microbial cells. The aim of this review is to examine our understanding of the predatory life cycle of M. xanthus. We will examine the multicellular structures formed during contact with prey, and the molecular mechanisms utilized by M. xanthus to detect and destroy prey cells. We will also examine our understanding of microbial predator-prey relationships and the prospects for how bacterial predation mechanisms can be exploited to generate new antimicrobial technologies.

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.

Independence and interdependence of Dif and Frz chemosensory pathways in Myxococcus xanthus chemotaxis

Dif and Frz, two Myxococcus xanthus chemosensory pathways, are required in phosphatidylethanolamine (PE) chemotaxis for excitation and adaptation respectively. DifA and FrzCD, the homologues of methyl-accepting chemoreceptors in the two pathways, were examined for methylation in the context of chemotaxis and inter-pathway interactions. Evidence indicates that DifA may not undergo methylation, but signals transmitting through DifA do modulate FrzCD methylation. Results also revealed that M. xanthus possesses Dif-dependent and Dif-independent PE-sensing mechanisms. Previous studies showed that FrzCD methylation is decreased by negative chemostimuli but increased by attractants such as PE. Results here demonstrate that the Dif-dependent sensory mechanism suppresses the increase in FrzCD methylation in attractant response and elevates FrzCD methylation upon negative stimulation. In other words, FrzCD methylation is governed by opposing forces from Dif-dependent and Dif-independent sensing mechanisms. We propose that the Dif-independent but Frz-dependent PE sensing leads to increases in FrzCD methylation and subsequent adaptation, while the Dif-dependent PE signalling suppresses or diminishes the increase in FrzCD methylation to decelerate or delay adaptation. We contend that these antagonistic interactions are crucial for effective chemotaxis in this gliding bacterium to ensure that adaptation does not occur too quickly relative to the slow speed of M. xanthus movement.

The mosaic genome of Anaeromyxobacter dehalogenans strain 2CP-C suggests an aerobic common ancestor to the delta-proteobacteria

Anaeromyxobacter dehalogenans strain 2CP-C is a versaphilic delta-Proteobacterium distributed throughout many diverse soil and sediment environments. 16S rRNA gene phylogenetic analysis groups A. dehalogenans together with the myxobacteria, which have distinguishing characteristics including strictly aerobic metabolism, sporulation, fruiting body formation, and surface motility. Analysis of the 5.01 Mb strain 2CP-C genome substantiated that this organism is a myxobacterium but shares genotypic traits with the anaerobic majority of the delta-Proteobacteria (i.e., the Desulfuromonadales). Reflective of its respiratory versatility, strain 2CP-C possesses 68 genes coding for putative c-type cytochromes, including one gene with 40 heme binding motifs. Consistent with its relatedness to the myxobacteria, surface motility was observed in strain 2CP-C and multiple types of motility genes are present, including 28 genes for gliding, adventurous (A-) motility and 17 genes for type IV pilus-based motility (i.e., social (S-) motility) that all have homologs in Myxococcus xanthus. Although A. dehalogenans shares many metabolic traits with the anaerobic majority of the delta-Proteobacteria, strain 2CP-C grows under microaerophilic conditions and possesses detoxification systems for reactive oxygen species. Accordingly, two gene clusters coding for NADH dehydrogenase subunits and two cytochrome oxidase gene clusters in strain 2CP-C are similar to those in M. xanthus. Remarkably, strain 2CP-C possesses a third NADH dehydrogenase gene cluster and a cytochrome cbb₃ oxidase gene cluster, apparently acquired through ancient horizontal gene transfer from a strictly anaerobic green sulfur bacterium. The mosaic nature of the A. dehalogenans strain 2CP-C genome suggests that the metabolically versatile, anaerobic members of the delta-Proteobacteria may have descended from aerobic ancestors with complex lifestyles.

Chemotaxis mediated by NarX-FrzCD chimeras and nonadapting repellent responses in Myxococcus xanthus

Myxococcus xanthus requires gliding motility for swarming and fruiting body formation. It uses the Frz chemosensory pathway to regulate cell reversals. FrzCD is a cytoplasmic chemoreceptor required for sensing effectors for this pathway. NarX is a transmembrane sensor for nitrate from Escherichia coli. In this study, two NarX-FrzCD chimeras were constructed to investigate M. xanthus chemotaxis: NazD(F) contains the N-terminal sensory module of NarX fused to the C-terminal signalling domain of FrzCD; NazD(R) is similar except that it contains a G51R mutation in the NarX domain known to reverse the signalling output of a NarX-Tar chimera to nitrate. We report that while nitrate had no effect on the wild type, it decreased the reversal frequency of M. xanthus expressing NazD(F) and increased that of M. xanthus expressing NazD(R). These results show that directional motility in M. xanthus can be regulated independently of cellular metabolism and physiology. Surprisingly, the NazD(R) strain failed to adapt to nitrate in temporal assays as did the wild type to known repellents. The lack of temporal adaptation to negative stimuli appears to be a general feature in M. xanthus chemotaxis. Thus, the appearance of biased movements by M. xanthus in repellent gradients is likely due to the inhibition of net translocation by repellents.

Proteins associated with the Myxococcus xanthus extracellular matrix

Fruiting body formation of Myxococcus xanthus, like biofilm formation of many other organisms, involves the production of an extracellular matrix (ECM). While the polysaccharide component has been studied, the protein component has been largely unexplored. Proteins associated with the ECM were solubilized from purified ECM by boiling with sodium dodecyl sulfate and were identified by liquid chromatography-tandem mass spectrometry of tryptic fragments. The ECM is enriched in proteins of novel function; putative functions were assigned for only 5 of the 21 proteins. Thirteen putative ECM proteins had lipoprotein secretion signals. The genes for many ECM proteins were disrupted in the wild-type (WT), fibA, and pilA backgrounds. Disruption of the MXAN4860 gene had no effect in the WT or fibA background but in the pilA background resulted in a 24-h delay in aggregation and sporulation compared to its parent. The results of this study show that the M. xanthus ECM proteome is diverse and novel.

Triple mutants uncover three new genes required for social motility in Myxococcus xanthus

The bacterium Myxococcus xanthus glides over surfaces using two different locomotive mechanisms, called S (social) and A (adventurous) motility that enable cells to move both as groups and as individuals. Neither mechanism involves flagella. The functions of these two motors are coordinated by the activity of a small Ras-like protein, encoded by the mglA gene. The results of previous studies of a second-site suppressor of the mglA-8 missense mutation masK-815 indicate that MglA interacts with a protein tyrosine kinase, MasK, to control social motility. Sequence analysis of the sites of 12 independent insertions of the transposon magellan-4 that result in the loss of motility in an M. xanthus mglA-8 masK-815 double mutant shows that nine of these 12 insertions are in genes known to be required for S gliding motility. This result confirms that the masK-815 suppressor restores S but not A motility. Three of the 12 insertions define three new genes required for S motility and show that the attachment of heptose to the lipopolysaccharide inner core, an ortholog of the CheR methyltransferase, and a large protein with YD repeat motifs, are required for S motility. When these three insertions are backcrossed into an otherwise wild-type genetic background, their recombinants are found to have defects in S, but not, A motility. The spectrum of magellan-4 insertions that lead to the loss of S motility in the mglA-8 masK-815 double mutant background is different than that resulting from a previous mutant hunt starting with a different (A mutant) genetic background, suggesting that the number of genes required for S motility in M. xanthus is quite large.

FibA and PilA act cooperatively during fruiting body formation of Myxococcus xanthus

The extracellular matrix (ECM) of Myxococcus xanthus is essential for social (S-) motility and fruiting body formation. An ECM-bound protein, FibA, is homologous to M4 zinc metalloproteases and is important for stimulation by a phosphatidylethanolamine (PE) chemoattractant and for formation of discrete aggregation foci. In this work, we demonstrate that a correlation exists between a reduced ability to respond to PE and the observed defects in fruiting body morphogenesis. Furthermore, the fibA aggregation defect is accentuated by the absence of either PilA, the structural subunit of type IV pili, or DifD, a chemosensory response regulator. The inability to form fruiting bodies is not due to a loss of S-motility, but rather the loss of PilA and pili as pilT fibA mutants form fruiting bodies. The FibA active site residue E342 is important for fruiting body morphogenesis in the absence of PilA. Mutants exhibiting defects in fruiting body morphogenesis also produce fewer viable spores. It is proposed that FibA and PilA act as extracellular sensors for developmental signals.

Phospholipid directed motility of surface-motile bacteria

Myxococcus xanthus is a surface-motile bacterium that has adapted at least one chemosensory system to allow directed movement towards the slowly diffusible lipid phosphatidylethanolamine (PE). The Dif chemosensory pathway is remarkable because it has at least three inputs coupled to outputs that control extracellular matrix (ECM) production and lipid chemotaxis. The methyl-accepting chemotaxis protein, DifA, has two different sensor inputs that have been localized by mutagenesis. The Dif chemosensory pathway employs a novel protein that slows adaptation. Lipid chemotaxis may play important roles in the M. xanthus life cycle where prey-specific and development-specific attractants have been identified. Lipid chemotaxis may also be an important mechanism for locating nutrients by lung pathogens such as Pseudomonas aeruginosa.

Novel lipids in Myxococcus xanthus and their role in chemotaxis

Organisms that colonize solid surfaces, like Myxococcus xanthus, use novel signalling systems to organize multicellular behaviour. Phosphatidylethanolamine (PE) containing the fatty acid 16:1omega5 (Delta11) elicits a chemotactic response. The phenomenon was examined by observing the effects of PE species with varying fatty acid pairings. Wild-type M. xanthus contains 17 different PE species under vegetative conditions and 19 at the midpoint of development; 13 of the 17 have an unsaturated fatty acid at the sn-1 position, a novelty among Proteobacteria. Myxococcus xanthus has two glycerol-3-phosphate acyltransferase (PlsB) homologues which add the sn-1 fatty acid. Each produces PE with 16:1 at the sn-1 position and supports growth and fruiting body development. Deletion of plsB1 (MXAN3288) results in more dramatic changes in PE species distribution than deletion of plsB2 (MXAN1675). PlsB2 has a putative N-terminal eukaryotic fatty acid reductase domain and may support both ether lipid synthesis and PE synthesis. Disruption of a single sn-2 acyltransferase homologue (PlsC, of which M. xanthus contains five) results in minor changes in membrane PE. Derivatization of purified PE extracts with dimethyldisulfide was used to determine the position of the double bonds in unsaturated fatty acids. The results suggest that Delta5 and Delta11 desaturases may create the double bonds after synthesis of the fatty acid. Phosphatidylethanolamine enriched for 16:1 at the sn-1 position stimulates chemotaxis more strongly than PE with 16:1 enriched at the sn-2 position. It appears that the deployment of a rare fatty acid (16:1omega5) at an unusual position (sn-1) has facilitated the evolution of a novel cell signal.

Type IV pili function upstream of the Dif chemotaxis pathway in Myxococcus xanthus EPS regulation

The developmental bacterium Myxococcus xanthus utilizes gliding motility to aggregate during the formation of multicellular fruiting bodies. The social (S) component of M. xanthus gliding motility requires at least two extracellular surface structures, type IV pili (Tfp) and the fibril polysaccharide or exopolysaccharide (EPS). Retraction of Tfp is proposed to power S motility and EPS from neighbouring cells is suggested to provide an anchor and trigger for Tfp retraction. The production of EPS in M. xanthus is regulated in part by the Dif chemosensory pathway; however, the input signal for the Dif pathway in EPS regulation remains to be uncovered. Using a genetic approach combined with quantitative and qualitative analysis, we demonstrate here that Tfp function upstream of the Dif proteins in regulating EPS production. The requirement of Tfp for the production of EPS was verified using various classes of Tfp mutants. Construction and examination of double and triple mutants indicated that mutations in dif are epistatic to those in pil. Furthermore, extracellular complementation between various Tfp and dif mutants suggests that Tfp, instead of being signals, may constitute the sensor or part of the sensor responsible for mediating signal input into the Dif pathway. We propose that S motility involves a regulatory loop in which EPS triggers Tfp retraction and Tfp provide proximity signals to the Dif pathway to modulate EPS production.

Cohesion-defective mutants of Myxococcus xanthus

Cohesion of Myxococcus xanthus cells involves interaction of a cell surface cohesin with a component of the extracellular matrix. In this work, two previously isolated cohesion-defective (fbd) mutants were characterized. The fbdA and fbdB genes do not encode the cohesins but are necessary for their production. Both mutants produce type IV pili, suggesting that PilA is not a major cohesin.