MotAB-like machinery drives the movement of MreB filaments during bacterial gliding motility

MreB is a bacterial actin that is important for cell shape and cell wall biosynthesis in many bacterial species. MreB also plays crucial roles in Myxococcus xanthus gliding motility, but the underlying mechanism remains unknown. Here we tracked the dynamics of single MreB particles in M. xanthus using single-particle tracking photoactivated localization microscopy. We found that a subpopulation of MreB particles moves rapidly along helical trajectories, similar to the movements of the MotAB-like gliding motors. The rapid MreB motion was stalled in the mutants that carried truncated gliding motors. Remarkably, M. xanthus MreB moves one to two orders of magnitude faster than its homologs that move along with the cell wall synthesis machinery in Bacillus subtilis and Escherichia coli, and this rapid movement was not affected by the inhibitors of cell wall biosynthesis. Our results show that in M. xanthus, MreB provides a scaffold for the gliding motors while the gliding machinery drives the movement of MreB filaments, analogous to the interdependent movements of myosin motors and actin in eukaryotic cells.

Novel mechanisms power bacterial gliding motility

For many bacteria, motility is essential for survival, growth, virulence, biofilm formation and intra/interspecies interactions. Since natural environments differ, bacteria have evolved remarkable motility systems to adapt, including swimming in aqueous media, and swarming, twitching and gliding on solid and semi-solid surfaces. Although tremendous advances have been achieved in understanding swimming and swarming motilities powered by flagella, and twitching motility powered by Type IV pili, little is known about gliding motility. Bacterial gliders are a heterogeneous group containing diverse bacteria that utilize surface motilities that do not depend on traditional flagella or pili, but are powered by mechanisms that are less well understood. Recently, advances in our understanding of the molecular machineries for several gliding bacteria revealed the roles of modified ion channels, secretion systems and unique machinery for surface movements. These novel mechanisms provide rich source materials for studying the function and evolution of complex microbial nanomachines. In this review, we summarize recent findings made on the gliding mechanisms of the myxobacteria, flavobacteria and mycoplasmas.

The lethal cargo of Myxococcus xanthus outer membrane vesicles

Myxococcus xanthus is a bacterial micro-predator known for hunting other microbes in a wolf pack-like manner. Outer membrane vesicles (OMVs) are produced in large quantities by M. xanthus and have a highly organized structure in the extracellular milieu, sometimes occurring in chains that link neighboring cells within a biofilm. OMVs may be a vehicle for mediating wolf pack activity by delivering hydrolytic enzymes and antibiotics aimed at killing prey microbes. Here, both the protein and small molecule cargo of the OMV and membrane fractions of M. xanthus were characterized and compared. Our analysis indicates a number of proteins that are OMV-specific or OMV-enriched, including several with putative hydrolytic function. Secondary metabolite profiling of OMVs identifies 16 molecules, many associated with antibiotic activities. Several hydrolytic enzyme homologs were identified, including the protein encoded by MXAN_3564 (mepA), an M36 protease homolog. Genetic disruption of mepA leads to a significant reduction in extracellular protease activity suggesting MepA is part of the long-predicted (yet to date undetermined) extracellular protease suite of M. xanthus.

Bacteria that glide with helical tracks

Many bacteria glide smoothly on surfaces, despite having no discernable propulsive organelles on their surface. Recent experiments with Myxococcus xanthus and Flavobacterium johnsoniae show that both of these distantly related bacterial species glide using proteins that move in helical tracks, albeit with significantly different motility mechanisms. Both species utilize proton-motive force for movement. Although the motors that power gliding in M. xanthus have been identified, the F. johnsoniae motors remain to be discovered.

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.

Chemosensory signaling controls motility and subcellular polarity in Myxococcus xanthus

Myxococcus xanthus is a model system for the study of dynamic protein localization and cell polarity in bacteria. M. xanthus cells are motile on solid surfaces enabled by two forms of motility. Motility is controlled by the Che-like Frz pathway, which is essential for fruiting body formation and differentiation. The Frz signal is mediated by a GTPase/GAP protein pair that establishes cell polarity and directs the motility systems. Pilus driven motility at the leading pole of the cell requires dynamic localization of two ATPases and the coordinated production of EPS synthesis. Gliding motility requires dynamic movement of large protein complexes, but the mechanism by which this system generates propulsive force is still an active area of investigation.

Uncovering the mystery of gliding motility in the myxobacteria

Bacterial gliding motility is the smooth movement of cells on solid surfaces unaided by flagella or pili. Many diverse groups of bacteria exhibit gliding, but the mechanism of gliding motility has remained a mystery since it was first observed more than a century ago. Recent studies on the motility of Myxococcus xanthus, a soil myxobacterium, suggest a likely mechanism for gliding in this organism. About forty M. xanthus genes were shown to be involved in gliding motility, and some of their protein products were labeled and localized within cells. These studies suggest that gliding motility in M. xanthus involves large multiprotein structural complexes, regulatory proteins, and cytoskeletal filaments. In this review, we summarize recent experiments that provide the basis for this emerging view of M. xanthus motility. We also discuss alternative models for gliding.

FrzS regulates social motility in Myxococcus xanthus by controlling exopolysaccharide production

Myxococcus xanthus Social (S) motility occurs at high cell densities and is powered by the extension and retraction of Type IV pili which bind ligands normally found in matrix exopolysaccharides (EPS). Previous studies showed that FrzS, a protein required for S-motility, is organized in polar clusters that show pole-to-pole translocation as cells reverse their direction of movement. Since the leading cell pole is the site of both the major FrzS cluster and type IV pilus extension/retraction, it was suggested that FrzS might regulate S-motility by activating pili at the leading cell pole. Here, we show that FrzS regulates EPS production, rather than type IV pilus function. We found that the frzS phenotype is distinct from that of Type IV pilus mutants such as pilA and pilT, but indistinguishable from EPS mutants, such as epsZ. Indeed, frzS mutants can be rescued by the addition of purified EPS, 1% methylcellulose, or co-culturing with wildtype cells. Our data also indicate that the cell density requirement in S-motility is likely a function of the ability of cells to construct functional multicellular clusters surrounding an EPS core.

Oral-derived bacterial flora defends its domain by recognizing and killing intruders--a molecular analysis using Escherichia coli as a model intestinal bacterium

Within the same human gastrointestinal tract, substantial differences in the bacterial species that inhabit oral cavity and intestinal tract have been noted. Previous research primarily attributed the differences to the influences of host environments and nutritional availabilities ("host habitat" effect). Our recent study indicated that, other than the host habitat effect, an existing microbial community could impose a selective pressure on incoming foreign bacterial species independent of host-mediated selection ("community selection" effect). In this study, we employed in vitro microbial floras representing microorganisms that inhabit the oral cavities and intestinal tract of mice in combination with Escherichia coli as a model intestinal bacterium and demonstrated that E. coli displays a striking community preference. It thrived when introduced into the intestinal microbial community and survived poorly in the microbial flora of foreign origin (oral community). A more detailed examination of this phenomenon showed that the oral community produced oxygen-free radicals in the presence of wild-type E. coli while mutants deficient in lipopolysaccharides (LPS) did not trigger significant production of these cell-damaging agents. Furthermore, mutants of E. coli defective in the oxidative stress response experienced a more drastic reduction in viability when cocultivated with the oral flora, while the exogenous addition of the antioxidant vitamin C was able to rescue it. We concluded that the oral-derived microbial community senses the E. coli LPS and kills the bacterium with oxygen-free radicals. This study reveals a new mechanism of community invasion resistance employed by established microflora to defend their domains.

In vitro communities derived from oral and gut microbial floras inhibit the growth of bacteria of foreign origins

The gastrointestinal (GI) tract is home to trillions of microbes. Within the same GI tract, substantial differences in the bacterial species that inhabit the oral cavity and intestinal tract have been noted. While the influence of host environments and nutritional availability in shaping different microbial communities is widely accepted, we hypothesize that the existing microbial flora also plays a role in selecting the bacterial species that are being integrated into the community. In this study, we used cultivable microbial communities isolated from different parts of the GI tract of mice (oral cavity and intestines) as a model system to examine this hypothesis. Microbes from these two areas were harvested and cultured using the same nutritional conditions, which led to two distinct microbial communities, each with about 20 different species as revealed by PCR-based denaturing gradient gel electrophoresis analysis. In vitro community competition assays showed that the two microbial floras exhibited antagonistic interactions toward each other. More interestingly, all the original isolates tested and their closely related species displayed striking community preferences: They persisted when introduced into the bacterial community of the same origin, while their viable count declined more than three orders of magnitude after 4 days of coincubation with the microbial flora of foreign origin. These results suggest that an existing microbial community might impose a selective pressure on incoming foreign bacterial species independent of host selection. The observed inter-flora interactions could contribute to the protective effect of established microbial communities against the integration of foreign bacteria to maintain the stability of the existing communities.

A multi-protein complex from Myxococcus xanthus required for bacterial gliding motility

Myxococcus xanthus moves by gliding motility powered by Type IV pili (S-motility) and a second motility system, A-motility, whose mechanism remains elusive despite the identification of approximately 40 A-motility genes. In this study, we used biochemistry and cell biology analyses to identify multi-protein complexes associated with A-motility. Previously, we showed that the N-terminal domain of FrzCD, the receptor for the frizzy chemosensory pathway, interacts with two A-motility proteins, AglZ and AgmU. Here we characterized AgmU, a protein that localized to both the periplasm and cytoplasm. On firm surfaces, AgmU-mCherry colocalized with AglZ as distributed clusters that remained fixed with respect to the substratum as cells moved forward. Cluster formation was favoured by hard surfaces where A-motility is favoured. In contrast, AgmU-mCherry clusters were not observed on soft agar surfaces or when cells were in large groups, conditions that favour S-motility. Using glutathione-S-transferase affinity chromatography, AgmU was found to interact either directly or indirectly with multiple A-motility proteins including AglZ, AglT, AgmK, AgmX, AglW and CglB. These proteins, important for the correct localization of AgmU and AglZ, appear to be organized as a motility complex, spanning the cytoplasm, inner membrane and the periplasm. Identification of this complex may be important for uncovering the mechanism of A-motility.

AglZ regulates adventurous (A-) motility in Myxococcus xanthus through its interaction with the cytoplasmic receptor, FrzCD

Myxococcus xanthus moves by gliding motility powered by type IV pili (S-motility) and distributed motor complexes (A-motility). The Frz chemosensory pathway controls reversals for both motility systems. However, it is unclear how the Frz pathway can communicate with these different systems. In this article, we show that FrzCD, the Frz pathway receptor, interacts with AglZ, a protein associated with A-motility. Affinity chromatography and cross-linking experiments showed that the FrzCD-AglZ interaction occurs between the uncharacterized N-terminal region of FrzCD and the N-terminal pseudo-receiver domain of AglZ. Fluorescence microscopy showed AglZ-mCherry and FrzCD-GFP localized in clusters that occupy different positions in cells. To study the role of the Frz system in the regulation of A-motility, we constructed aglZ frzCD double mutants and aglZ frzCD pilA triple mutants. To our surprise, these mutants, predicted to show no A-motility (A-S+) or no motility at all (A-S-), respectively, showed restored A-motility. These results indicate that AglZ modulates a FrzCD activity that inhibits A-motility. We hypothesize that AglZ-FrzCD interactions are favoured when cells are isolated and moving by A-motility and inhibited when S-motility predominates and A-motility is reduced.

Site-specific receptor methylation of FrzCD in Myxococcus xanthus is controlled by a tetra-trico peptide repeat (TPR) containing regulatory domain of the FrzF methyltransferase

Myxococcus xanthus is a gliding bacterium with a complex life cycle that includes swarming, predation and fruiting body formation. Directed movements in M. xanthus are regulated by the Frz chemosensory system, which controls cell reversals. The Frz pathway requires the activity of FrzCD, a cytoplasmic methyl-accepting chemotaxis protein, and FrzF, a methyltransferase (CheR) containing an additional domain with three tetra trico-peptide repeats (TPRs). To investigate the role of the TPRs in FrzCD methylation, we used full-length FrzF and FrzF lacking its TPRs (FrzF^CheR) to methylate FrzCD in vitro. FrzF methylated FrzCD on a single residue, E182, while FrzF^CheR methylated FrzCD on three residues, E168, E175 and E182, indicating that the TPRs regulate site-specific methylation. E168 and E182 were predicted consensus methylation sites, but E175 is methylated on an HE pair. To determine the roles of these sites in vivo, we substituted each methylatable glutamate with either an aspartate or an alanine residue and determined the impact of the point mutants on single cell reversals, swarming and fruiting body formation. Single, double and triple methylation site mutants revealed that each site played a unique role in M. xanthus behaviour and that the pattern of receptor methylation determined receptor activity. This work also shows that methylation can both activate and inactivate the receptor.

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.

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.

Polarity of motility systems in Myxococcus xanthus

Myxococcus xanthus is a gliding bacterium that contains two motility systems: S-motility, powered by polar type IV pili, and A-motility, powered by uncharacterized motors and adhesion complexes. The localization and coordination of the two motility engines is essential for directed motility as cells move forward and reverse. During cell reversals, the polarity and localization of motility proteins are rapidly inverted, rendering this system a fascinating example of dynamic protein localization.

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.

The motors powering A-motility in Myxococcus xanthus are distributed along the cell body

Two models have been proposed to explain the adventurous gliding motility of Myxococcus xanthus: (i) polar secretion of slime and (ii) an unknown motor that uses cell surface adhesion complexes that form periodic attachments along the cell length. Gliding movements of the leading poles of cephalexin-treated filamentous cells were observed but not equivalent movements of the lagging poles. This demonstrates that the adventurous-motility motors are not confined to the rear of the cell.

Two localization motifs mediate polar residence of FrzS during cell movement and reversals of Myxococcus xanthus

Myxococcus xanthus utilizes two motility systems for surface locomotion: A-motility and S-motility. S-motility is mediated by extension and retraction of type IV pili. Cells exhibiting S-motility periodically reverse by switching the assembly of type IV pili from the old leading pole to the new leading pole. These cellular reversals involve regulated pole-to-pole oscillations of the FrzS protein. We constructed and characterized in-frame deletion mutations in several FrzS domains to determine their roles in protein localization. We found that FrzS has distinct domains required for residence at the leading cell pole, pole-to-pole transport and lagging cell pole. Our results are consistent with a model whereby S-motility reversals are mediated by a protein translocation system that delivers motility proteins such as FrzS from the leading cell pole to the lagging cell pole.

An atypical receiver domain controls the dynamic polar localization of the Myxococcus xanthus social motility protein FrzS

The Myxococcus xanthus FrzS protein transits from pole-to-pole within the cell, accumulating at the pole that defines the direction of movement in social (S) motility. Here we show using atomic-resolution crystallography and NMR that the FrzS receiver domain (RD) displays the conserved switch Tyr102 in an unusual conformation, lacks the conserved Asp phosphorylation site, and fails to bind Mg(2+) or the phosphoryl analogue, Mg(2+) x BeF(3). Mutation of Asp55, closest to the canonical site of RD phosphorylation, showed no motility phenotype in vivo, demonstrating that phosphorylation at this site is not necessary for domain function. In contrast, the Tyr102Ala and His92Phe substitutions on the canonical output face of the FrzS RD abolished S-motility in vivo. Single-cell fluorescence microscopy measurements revealed a striking mislocalization of these mutant FrzS proteins to the trailing cell pole in vivo. The crystal structures of the mutants suggested that the observed conformation of Tyr102 in the wild-type FrzS RD is not sufficient for function. These results support the model that FrzS contains a novel 'pseudo-receiver domain' whose function requires recognition of the RD output face but not Asp phosphorylation.

Evidence that focal adhesion complexes power bacterial gliding motility

The bacterium Myxococcus xanthus has two motility systems: S motility, which is powered by type IV pilus retraction, and A motility, which is powered by unknown mechanism(s). We found that A motility involved transient adhesion complexes that remained at fixed positions relative to the substratum as cells moved forward. Complexes assembled at leading cell poles and dispersed at the rear of the cells. When cells reversed direction, the A-motility clusters relocalized to the new leading poles together with S-motility proteins. The Frz chemosensory system coordinated the two motility systems. The dynamics of protein cluster localization suggest that intracellular motors and force transmission by dynamic focal adhesions can power bacterial motility.

Aggregation during fruiting body formation in Myxococcus xanthus is driven by reducing cell movement

When starved, Myxococcus xanthus cells assemble themselves into aggregates of about 10(5) cells that grow into complex structures called fruiting bodies, where they later sporulate. Here we present new observations on the velocities of the cells, their orientations, and reversal rates during the early stages of fruiting body formation. Most strikingly, we find that during aggregation, cell velocities slow dramatically and cells orient themselves in parallel inside the aggregates, while later cell orientations are circumferential to the periphery. The slowing of cell velocity, rather than changes in reversal frequency, can account for the accumulation of cells into aggregates. These observations are mimicked by a continuous agent-based computational model that reproduces the early stages of fruiting body formation. We also show, both experimentally and computationally, how changes in reversal frequency controlled by the Frz system mutants affect the shape of these early fruiting bodies.

Two Ser/Thr protein kinases essential for efficient aggregation and spore morphogenesis in Myxococcus xanthus

Myxococcus xanthus has a complex life cycle that involves vegetative growth and development. Previously, we described the espAB locus that is involved in timing events during the initial stages of fruiting body formation. Deletion of espA caused early aggregation and sporulation, whereas deletion of espB caused delayed aggregation and sporulation resulting in reduced spore yields. In this study, we describe two genes, pktA5 and pktB8, that flank the espAB locus and encode Ser/Thr protein kinase (STPK) homologues. Cells deficient in pktA5 or pktB8 formed translucent mounds and produced low spore yields, similar in many respects to espB mutants. Double mutant analysis revealed that espA was epistatic to pktA5 and pktB8 with respect to aggregation and fruiting body morphology, but that pktA5 and pktB8 were epistatic to espA with respect to sporulation efficiency. Expression profiles of pktA5-lacZ and pktB8-lacZ fusions and Western blot analysis showed that the STPKs are expressed under vegetative and developmental conditions. In vitro kinase assays demonstrated that the RD kinase, PktA5, autophosphorylated on threonine residue(s) and phosphorylated the artificial substrate, myelin basic protein. In contrast, autophosphorylation of the non-RD kinase, PktB8, was not observed in vitro; however, the phenotype of a pktB8 kinase-dead point mutant resembled the pktB8 deletion mutant, indicating that this residue was important for function and that it likely functions as a kinase in vivo. Immunoprecipitation of Tap-tagged PktA5 and PktB8 revealed an interaction with EspA during development in M. xanthus. These results, taken together, suggest that PktA5 and PktB8 are STPKs that function during development by interacting with EspA and EspB to regulate M. xanthus development.

Differential effects of chemoreceptor methylation-domain mutations on swarming and development in the social bacterium Myxococcus xanthus

The soil bacterium Myxococcus xanthus is a model organism for the study of multicellular behaviour and development in bacteria. M. xanthus cells move on solid surfaces by gliding motility, periodically reversing their direction of movement. Motility is co-ordinated to allow cells to effectively feed on macromolecules or prey bacteria when nutrients are plentiful and to form developmental fruiting bodies when nutrients are limiting. The Frz signal transduction pathway regulates cellular movements by modulating cell reversal frequency. Input to the Frz pathway is controlled by the cytoplasmic receptor, FrzCD, a methyl-accepting chemotaxis protein (MCP). FrzCD lacks the transmembrane and periplasmic domains common to MCPs but contains a unique N-terminal domain, the predicted ligand-binding domain. As deletion of the N-terminal domain of FrzCD only results in minor defects in motility, we investigated the possibility that the methylation of the conserved C-terminal domain of FrzCD plays a central role in regulating the pathway. For this study, each of the potential methylation sites of FrzCD were systematically modified by site-directed mutagenesis, substituting glutamine/glutamate pairs for alanines. Four of the seven mutations produced dramatic phenotypes; two of the mutations had a stimulatory effect on the pathway, as evidenced by cells hyper-reversing, whereas another two had an inhibitory effect, causing these cells to rarely reverse. These four mutants displayed defects in vegetative swarming and developmental aggregation. These results suggests a model in which the methylation domain can both activate and inhibit the Frz pathway depending on which residues are methylated. The diversity of phenotypes suggests that specific modifications of FrzCD act to differentially regulate motility and developmental aggregation in M. xanthus.

Accordion waves in Myxococcus xanthus

Myxococcus xanthus are Gram-negative bacteria that glide on solid surfaces, periodically reversing their direction of movement. When starved, M. xanthus cells organize their movements into waves of cell density that sweep over the colony surface. These waves are unique: Although they appear to interpenetrate, they actually reflect off one another when they collide, so that each wave crest oscillates back and forth with no net displacement. Because the waves reflect the coordinated back and forth oscillations of the individual bacteria, we call them "accordion" waves. The spatial oscillations of individuals are a manifestation of an internal biochemical oscillator, probably involving the Frz chemosensory system. These internal "clocks," each of which is quite variable, are synchronized by collisions between individual cells using a contact-mediated signal-transduction system. The result of collision signaling is that the collective spatial behavior is much less variable than the individual oscillators. In this work, we present experimental observations in which individual cells marked with GFP can be followed in groups of unlabeled cells in monolayer cultures. These data, together with an agent-based computational model demonstrate that the only properties required to explain the ripple patterns are an asymmetric biochemical limit cycle that controls direction reversals and asymmetric contact-induced signaling between cells: Head-to-head signaling is stronger than head-to-tail signaling. Together, the experimental and computational data provide new insights into how populations of interacting oscillators can synchronize and organize spatially to produce morphogenetic patterns that may have parallels in higher organisms.

Four unusual two-component signal transduction homologs, RedC to RedF, are necessary for timely development in Myxococcus xanthus

We identified a cluster of four two-component signal transduction genes that are necessary for proper progression of Myxococcus xanthus through development. redC to redF mutants developed and sporulated early, resulting in small, numerous, and disorganized fruiting bodies. Yeast two-hybrid analyses suggest that RedCDEF act in a single signaling pathway. The previously identified espA gene displays a phenotype similar to that of redCDEF. However, combined mutants defective in espA redCDEF exhibited a striking additive developmental phenotype, suggesting that EspA and RedC to RedF play independent roles in controlling developmental progression.

Regulated pole-to-pole oscillations of a bacterial gliding motility protein

Little is known about directed motility of bacteria that move by type IV pilus-mediated (twitching) motility. Here, we found that during periodic cell reversals of Myxoccocus xanthus, type IV pili were disassembled at one pole and reassembled at the other pole. Accompanying these reversals, FrzS, a protein required for directed motility, moved in an oscillatory pattern between the cell poles. The frequency of the oscillations was controlled by the Frz chemosensory system, which is essential for directed motility. Pole-to-pole migration of FrzS appeared to involve movement along a filament running the length of the cell. FrzS dynamics may thus regulate cell polarity during directed motility.

EspC is involved in controlling the timing of development in Myxococcus xanthus

The espC null mutation caused accelerated aggregation and formation of tiny fruiting bodies surrounded by spores, which were also observed in the espA mutant and in CsgA-overproducing cells in Myxococcus xanthus. In addition, the espC mutant appeared to produce larger amounts of the complementary C-signal than the wild-type strain. These findings suggest that EspC is involved in controlling the timing of fruiting body development in M. xanthus.

Exopolysaccharide biosynthesis genes required for social motility in Myxococcus xanthus

Social (S)-motility in Myxococcus xanthus is a flagellum-independent gliding motility system that allows bacteria to move in groups on solid surfaces. S-motility has been shown to require type IV pili (TFP), exopolysaccharide (EPS; a component of fibrils) and lipopolysaccharide (LPS). Previously, information concerning EPS biogenesis in M. xanthus was lacking. In this study, we screened 5000 randomly mutagenized colonies for defects in S-motility and EPS and identified two genetic regions essential for EPS biogenesis: the EPS synthesis (eps) region and the EPS-associated (eas) region. Mutants with insertions in the eps and eas regions were defective in S-motility and fruiting body formation. These mutants failed to bind the dye calcofluor white, indicating that they lacked EPS; however, they retained normal TFP and LPS. Analysis of the eps locus showed several open reading frames (ORFs) that encode homologues to glycosyltransferases, glucanases and EPS transporters as well as regulatory proteins; the eas locus contains two ORFs: one exhibits homology to hypothetical proteins with a conserved domain of unknown function and the other displays no apparent homology to other proteins in the database. Further genetic mutagenesis analysis indicates that the whole eps region is involved in the biosynthesis of fibrils and fibril EPS. The operon at the proximal end of the eps region was analysed by generating in-frame deletion mutations. These mutants showed varying degrees of defects in the bacterium's ability to produce EPS or perform EPS-related functions, confirming the involvement of these genes in M. xanthus EPS biogenesis.

The Che4 pathway of Myxococcus xanthus regulates type IV pilus-mediated motility

Myxococcus xanthus co-ordinates cell movement during its complex life cycle using multiple chemotaxis-like signal transduction pathways. These pathways regulate both type IV pilus-mediated social (S) motility and adventurous (A) motility. During a search for new chemoreceptors, we identified the che4 operon, which encodes homologues to a MCP (methyl-accepting chemotaxis protein), two CheWs, a hybrid CheA-CheY, a response regulator and a CheR. Deletion of the che4 operon did not cause swarming or developmental defects in either the wild-type (A(+)S(+)) strain or in a strain sustaining only A motility (A(+)S(-)). However, in a strain displaying only S motility (A(-)S(+)), deletion of the che4 operon or the gene encoding the response regulator, cheY4, caused enhanced vegetative swarming and prevented aggregation and sporulation. In contrast, deletion of mcp4 caused reduced vegetative swarming and enhanced development compared with the parent strain. Single-cell analysis of the motility of the A(-)S(+) parent strain revealed a previously unknown inverse correlation between velocity and reversal frequency. Thus, cells that moved at higher velocities showed a reduced reversal frequency. This co-ordination of reversal frequency and velocity was lost in the mcp4 and cheY4 mutants. The structural components of the S motility apparatus were unaffected in the che4 mutants, suggesting that the Che4 system affects reversal frequency of cells by modulating the function of the type IV pilus.

Analysis of the Frz signal transduction system of Myxococcus xanthus shows the importance of the conserved C-terminal region of the cytoplasmic chemoreceptor FrzCD in sensing signals

The Frz chemosensory system controls directed motility in Myxococcus xanthus by regulating cellular reversal frequency. M. xanthus requires the Frz system for vegetative swarming on rich media and for cellular aggregation during fruiting body formation on starvation media. The Frz signal transduction pathway is formed by proteins that share homology with chemotaxis proteins from enteric bacteria, which are encoded in the frzA-F putative operon and the divergently transcribed frzZ gene. FrzCD, the Frz system chemoreceptor, contains a conserved C-terminal module present in methyl-accepting chemotaxis proteins (MCPs); but, in contrast to most MCPs, FrzCD is localized in the cytoplasm and the N-terminal region of FrzCD does not contain transmembrane or sensing domains, or even a linker region. Previous work on the Frz system was limited by the unavailability of deletion strains. To understand better how the Frz system functions, we generated a series of in-frame deletions in each of the frz genes as well as regions encoding the N-terminal portion of FrzCD. Analysis of mutants containing these deletions showed that FrzCD (MCP), FrzA (CheW) and FrzE (CheA-CheY) control vegetative swarming, responses to repellents and directed movement during development, thus constituting the core components of the Frz pathway. FrzB (CheW), FrzF (CheR), FrzG (CheB) and FrzZ (CheY-CheY) are required for some but not all responses. Furthermore, deletion of approximately 25 amino acids from either end of the conserved C-terminal region of FrzCD results in a constitutive signalling state of FrzCD, which induces hyper-reversals with no net cell movement. Surprisingly, deletion of the N-terminal region of FrzCD shows only minor defects in swarming. Thus, signal input to the Frz system must be sensed by the conserved C-terminal module of FrzCD and not the usual N-terminal region. These results indicate an alternative mechanism for signal sensing with this cytoplasmic MCP.

Chemosensory regulation of developmental gene expression in Myxococcus xanthus

The delta-proteobacterium Myxococcus xanthus coordinates its motility during aggregation and fruiting body formation. While searching for chemotaxis genes in M. xanthus, we identified a third chemotaxis-like gene cluster, the che3 cluster, encoding homologs to two methyl-accepting chemotaxis proteins (MCPs), a CheW, a hybrid CheA, a CheB, a CheR, but no CheY. Mutations in mcp3A, mcp3B, and cheA3 did not show obvious defects in motility or chemotaxis but did affect the timing of entry into development. Mutations in these genes caused early aggregation of starving cells, even at low cell densities. Furthermore, these mutants showed pronounced overexpression of the developmentally regulated Tn5lac fusions Omega4403, Omega4411, and Omega4521 as well as overexpression of mbhA and tps, markers for peripheral rods and aggregating cells, respectively. Divergently transcribed from the che3 promoter region is another gene, crdA (chemosensory regulator of development), predicted to encode a transcriptional activator of sigma(54)-dependent promoters. To test the hypothesis that CrdA functions as the cognate response regulator for the histidine kinase CheA3, CrdA and CheA3 were assayed and found to interact strongly in the yeast two-hybrid system. Mutant analysis showed that crdA cells were delayed in development (12-24 h) and delayed in MbhA production relative to the wild type. An mcp3BcrdA double mutant displayed the crdA phenotype, indicating that crdA is epistatic to mcp3B. We conclude that the Che3 chemotaxis-like system functions to control developmental gene expression by regulating a sigma(54) transcriptional activator, CrdA.

Pph1 from Myxococcus xanthus is a protein phosphatase involved in vegetative growth and development

Myxococcus xanthus is a Gram-negative bacterium with a complex life cycle that includes vegetative swarming on rich medium and, upon starvation, aggregation to form fruiting bodies containing spores. Both of these behaviours require multiple Ser/Thr protein kinases. In this paper, we report the first Ser/Thr protein phosphatase gene, pph1, from M. xanthus. DNA sequence analysis of pph1 indicates that it encodes a protein of 254 residues (Mr = 28 308) with strong homology to eukaryotic PP2C phosphatases and that it belongs to a new group of bacterial protein phosphatases that are distinct from bacterial PP2C phosphatases such as RsbU, RsbX and SpoIIE. Recombinant His-tagged Pph1 was purified from Escherichia coli and shown to have Mn2+ or Mg2+ dependent, okadaic acid-resistant phosphatase activity on a synthetic phosphorylated peptide, RRA(pT)VA, indicating that Pph1 is a PP2C phosphatase. Pph1-expression was observed under both vegetative and developmental conditions, but peaked during early aggregation. A pph1 null mutant showed defects during late vegetative growth, swarming and glycerol spore formation. Under starvation-induced developmental conditions, the mutant showed reduced aggregation and failure to form fruiting bodies with viable spores. Using the yeast two-hybrid system, we have observed a strong interaction between Pph1 and the M. xanthus protein kinase Pkn5, a negative effector of development. These results suggest a functional link between a Pkn2-type protein kinase and a PP2C phosphatase.

Developmental aggregation of Myxococcus xanthus requires frgA, an frz-related gene

Myxococcus xanthus is a gram-negative bacterium which has a complex life cycle that includes multicellular fruiting body formation. Frizzy mutants are characterized by the formation of tangled filaments instead of hemispherical fruiting bodies on fruiting agar. Mutations in the frz genes have been shown to cause defects in directed motility, which is essential for both vegetative swarming and fruiting body formation. In this paper, we report the discovery of a new gene, called frgA (for frz-related gene), which confers a subset of the frizzy phenotype when mutated. The frgA null mutant showed reduced swarming and the formation of frizzy aggregates on fruiting agar. However, this mutant still displayed directed motility in a spatial chemotaxis assay, whereas the majority of frz mutants fail to show directed movements in this assay. Furthermore, the frizzy phenotype of the frgA mutant could be complemented extracellularly by wild-type cells or strains carrying non-frz mutations. The phenotype of the frgA mutant is similar to that of the abcA mutant and suggests that both of these mutants could be defective in the production or export of extracellular signals required for fruiting body formation rather than in the sensing of such extracellular signals. The frgA gene encodes a large protein of 883 amino acids which lacks homologues in the databases. The frgA gene is part of an operon which includes two additional genes, frgB and frgC. The frgB gene encodes a putative histidine protein kinase, and the frgC gene encodes a putative response regulator. The frgB and frgC null mutants, however, formed wild-type fruiting bodies.

Type IV pilus of Myxococcus xanthus is a motility apparatus controlled by the frz chemosensory system

Although flagella are the best-understood means of locomotion in bacteria [1], other bacterial motility mechanisms must exist as many diverse groups of bacteria move without the aid of flagella [2-4]. One unusual structure that may contribute to motility is the type IV pilus [5,6]. Genetic evidence indicates that type IV pili are required for social gliding motility (S-motility) in Myxococcus, and twitching motility in Pseudomonas and Neisseria [6,7]. It is thought that type IV pili may retract or rotate to bring about cellular motility [6,8], but there is no direct evidence for the role of pili in cell movements. Here, using a tethering assay, we obtained evidence that the type IV pilus of Myxococcus xanthus functions as a motility apparatus. Pili were required for M. xanthus cells to adhere to solid surfaces and to generate cellular movement using S-motility. Tethered cells were released from the surface at intervals corresponding to the reversal frequency of wild-type cells when gliding on a solid surface. Mutants defective in the control of directional movements and cellular reversals (frz mutants) showed altered patterns of adherence that correlate reversal frequencies with tethering. The behavior of the tethered cells was consistent with a model in which the pili are extruded from one cell pole, adhere to a surface, and then retract, pulling the cell in the direction of the adhering pili. Cellular reversals would result from the sites of pili extrusion switching from one cell pole to another and are controlled by the frz chemosensory system.

Social motility in Myxococcus xanthus requires FrzS, a protein with an extensive coiled-coil domain

Gliding motility in the developmental bacterium Myxococcus xanthus involves two genetically distinct motility systems, designated adventurous (A) and social (S). Directed motility responses, which facilitate both vegetative swarming and developmental aggregation, additionally require the 'frizzy' (Frz) signal transduction pathway. In this study, we have analysed a new gene (frzS), which is positioned upstream of the frzA-F operon. Insertion mutations in frzS caused both vegetative spreading and developmental defects, including 'frizzy' aggregates in the FB strain background. The 'frizzy' phenotype was previously considered to result only from defective directed motility responses. However, deletion of the frzS gene in an A-S+ motility background demonstrated that FrzS is a new component of the S-motility system, as the A-frzS double mutant was non-spreading (A-S-). Compared with known S-motility mutants, the frzS mutants appear similar to pilT mutants, in that both produce type IV pili, extracellular fibrils and lipopolysaccharide (LPS) O-antigen, and both agglutinate rapidly in a cohesion assay. The FrzS protein has an unusual domain composition for a bacterial protein. The N-terminal domain shows similarity to the receiver domains of the two-component response regulator proteins. The C-terminal domain is composed of up to 38 heptad repeats (a b c d e f g)38, in which residues at positions a and d are predominantly hydrophobic, whereas residues at positions e and g are predominantly charged. This periodic disposition of specific residues suggests that the domain forms a long coiled-coil structure, similar to those found in the alpha-fibrous proteins, such as myosin. Overexpression of this domain in Escherichia coli resulted in the formation of an unusual striated protein lattice that filled the cells. We speculate on the role that this novel protein could play in gliding motility.

Disruption of aldA influences the developmental process in Myxococcus xanthus

Previously, we identified a gene (aldA) from Myxococcus xanthus, which we suggested encoded the enzyme alanine dehydrogenase on the basis of similarity to known Ald protein sequences (M. J. Ward, H. Lew, A. Treuner-Lange, and D. R. Zusman, J. Bacteriol. 180:5668-5675, 1998). In this study, we have confirmed that aldA does encode a functional alanine dehydrogenase, since it catalyzes the reversible conversion of alanine to pyruvate and ammonia. Whereas an aldA gene disruption mutation did not significantly influence the rate of growth or spreading on a rich medium, AldA was required for growth on a minimal medium containing L-alanine as the major source of carbon. Under developmental conditions, the aldA mutation caused delayed aggregation in both wild-type (DZ2) and FB (DZF1) strains. Poorly formed aggregates and reduced levels of spores were apparent in the DZ2 aldA mutant, even after prolonged development.

Motility in Myxococcus xanthus and its role in developmental aggregation

The Frz signal transduction system of Myxococcus xanthus was originally thought to be a simple variation of the well-characterized Che system of the enteric bacteria. Recently, however, many additional Frz proteins, along with alternative signal transduction systems, have been discovered. Together these signal transduction pathways coordinate cell-cell behavior, permitting the complex interactions required for developmental aggregation and fruiting body formation.

Sporulation timing in Myxococcus xanthus is controlled by the espAB locus

The fruiting body development of Myxococcus xanthus consists of two separate but interacting pathways: one for aggregation of many cells to form raised mounds and the other for sporulation of individual cells into myxospores. Sporulation of individual cells normally occurs after mound formation, and is delayed at least 30 h after starvation under our laboratory conditions. This suggests that M. xanthus has a mechanism that monitors progress towards aggregation prior to triggering sporulation. A null mutation in a newly identified gene, espA (early sporulation), causes sporulation to occur much earlier compared with the wild type (16 h earlier). In contrast, a null mutation in an adjacent gene, espB, delays sporulation by about 16 h compared with the wild type. Interestingly, it appears that the espA mutant does not require raised mounds for sporulation. Many mutant cells sporulate outside the fruiting bodies. In addition, the mutant can sporulate, without aggregation into raised mounds, under some conditions in which cells normally do not form fruiting bodies. Based on these observations, it is hypothesized that EspA functions as an inhibitor of sporulation during early fruiting body development while cells are aggregating into raised mounds. The aggregation-independent sporulation of the espA mutant still requires starvation and high cell density. The espA and espB genes are expressed as an operon and their translations appear to be coupled. Expression occurs only under developmental conditions and does not occur during vegetative growth or during glycerol-induced sporulation. Sequence analysis of EspA indicates that it is a histidine protein kinase with a fork head-associated (FHA) domain at the N-terminus and a receiver domain at the C-terminus. This suggests that EspA is part of a two-component signal transduction system that regulates the timing of sporulation initiation.

Induction of beta-lactamase influences the course of development in Myxococcus xanthus

Myxococcus xanthus is a gram-negative bacterium that develops in response to starvation on a solid surface. The cells assemble into multicellular aggregates in which they differentiate from rod-shaped cells into spherical, environmentally resistant spores. Previously, we have shown that the induction of beta-lactamase is associated with starvation-independent sporulation in liquid culture (K. A. O'Connor and D. R. Zusman, Mol. Microbiol. 24:839-850, 1997). In this paper, we show that the chromosomally encoded beta-lactamase of M. xanthus is autogenously induced during development. The specific activity of the enzyme begins to increase during aggregation, before spores are detectable. The addition of inducers of beta-lactamase in M. xanthus, such as ampicillin, D-cycloserine, and phosphomycin, accelerates the onset of aggregation and sporulation in developing populations of cells. In addition, the exogenous induction of beta-lactamase allows M. xanthus to fruit on media containing concentrations of nutrients that are normally too high to support development. We propose that the induction of beta-lactamase is an integral step in the development of M. xanthus and that this induction is likely to play a role in aggregation and in the restructuring of peptidoglycan which occurs during the differentiation of spores. In support of this hypothesis, we show that exogenous induction of beta-lactamase can rescue aggregation and sporulation of certain mutants. Fruiting body spores from a rescued mutant are indistinguishable from wild-type fruiting body spores when examined by transmission electron microscopy. These results show that the signal transduction pathway leading to the induction of beta-lactamase plays an important role in aggregation and sporulation in M. xanthus.

AsgD, a new two-component regulator required for A-signalling and nutrient sensing during early development of Myxococcus xanthus

Myxococcus xanthus has a complex life cycle that includes fruiting body formation. One of the first stages in development has been called A-signalling. The asg (A-signalling) mutants have been proposed to be deficient in producing A-signal, resulting in development arresting at an early stage. In this paper, we report the identification of a new asg locus asgD. This locus appears to be involved in both environmental sensing and intercellular signalling. Expression of asgD was undetected during vegetative growth, but increased dramatically within 1 h of starvation. The AsgD protein is predicted to contain 773 amino acids and to be part of a two-component regulatory system because it has a receiver domain located at the N-terminus and a histidine protein kinase at the C-terminus. An asgD null mutant was defective in fruiting body formation and sporulation on CF medium. However, the defects of the mutant were complemented extracellularly when cells were mixed with wild-type strains or with bsgA, csgA, dsgA or esgA mutants, but were not complemented extracellularly by asgA, asgB or asgC mutants. In addition, the mutant was rescued by a subset of A-factor amino acids. Surprisingly, when the mutant was plated on stringent starvation medium rather than CF, cells were able to form fruiting bodies. Thus, it appears that AsgD is directly or indirectly involved in sensing nutritionally limiting conditions. The discovery of the asgD locus provides an important sensory transduction component of early development in M. xanthus.

Regulation of motility behavior in Myxococcus xanthus may require an extracytoplasmic-function sigma factor

Using interaction trap technology, we identified a putative extracytoplasmic-function (ECF) sigma factor (RpoE1) in Myxococcus xanthus, a bacterium which has a complex life cycle that includes fruiting body formation. The first domain of the response regulator protein FrzZ, a component of the Frz signal transduction system, was used as bait. Although the RpoE1 protein displayed no interactions with control proteins presented as bait, a weak interaction with a second M. xanthus response regulator (AsgA) was observed. While the specificity of the FrzZ-RpoE1 interaction therefore remains speculative, cloning and sequencing of the region surrounding rpoE1 localized it to a position downstream of the frzZ gene. A potential promoter site for binding of an ECF sigma factor was identified upstream of rpoE1, suggesting the gene may be autoregulated. However, primer extension studies suggested that transcription of rpoE1 occurs under both vegetative and developmental conditions from a sigma70-like promoter. Dot blot analysis of RNA preparations confirmed the low-level, constitutive expression of rpoE1 during both stages of the life cycle. Analysis of an insertion mutant also indicated a role for RpoE1 under both vegetative and developmental conditions, since swarming was reduced on nutrient-rich agar and developmental aggregation was effected under starvation conditions, especially at high cell densities. An insertion mutation introduced into the gene directly downstream of rpoE1 (orf5) did not result in either swarming or developmental aggregation defects, even though the gene is transcribed as part of the same operon. Therefore, we propose that this new ECF sigma factor could play a role in the transcriptional regulation of genes involved in motility behavior during both stages of the complex M. xanthus life cycle.

An ABC transporter plays a developmental aggregation role in Myxococcus xanthus

Myxococcus xanthus is a gram-negative bacterium which has a complex life cycle. Autochemotaxis, a process whereby cells release a self-generated signaling molecule, may be the principal mechanism facilitating directed motility in both the vegetative swarming and developmental aggregation stages of this life cycle. The process requires the Frz signal transduction system, including FrzZ, a protein which is composed of two domains, both showing homology to the enteric chemotaxis response regulator CheY. The first domain of FrzZ (FrzZ1), when expressed as bait in the yeast two-hybrid system and screened against a library, was shown to potentially interact with the C-terminal portion of a protein encoding an ATP-binding cassette (AbcA). The activation domain-AbcA fusion protein did not interact with the second domain of FrzZ (FrzZ2) or with two other M. xanthus response regulator-containing proteins presented as bait, suggesting that the FrzZ1-AbcA interaction may be specific. Cloning and sequencing of the upstream region of the abcA gene showed the ATP-binding cassette to be linked to a large hydrophobic, potentially membrane-spanning domain. This domain organization is characteristic of a subgroup of ABC transporters which perform export functions. Cloning and sequencing downstream of abcA indicated that the ABC transporter is at the start of an operon containing three open reading frames. An insertion mutation in the abcA gene resulted in cells displaying the frizzy aggregation phenotype, providing additional evidence that FrzZ and AbcA may be part of the same signal transduction pathway. Cells with mutations in genes downstream of abcA showed no developmental defects. Analysis of the proposed exporter role of AbcA in cell mixing experiments showed that the ABC transporter mutant could be rescued by extracellular complementation. We speculate that the AbcA protein may be involved in the export of a molecule required for the autochemotactic process.

Myxococcus xanthus displays Frz-dependent chemokinetic behavior during vegetative swarming

Myxococcus xanthus has been shown to utilize both directed (tactic) and undirected (kinetic) movements during different stages of its complex life cycle. We have used time-lapse video microscopic analysis to separate tactic and kinetic behaviors associated specifically with vegetatively swarming cells. Isolated individual cells separated by a thin agar barrier from mature swarms showed significant increases in gliding velocity compared to that of similar cells some distance from the swarm. This orthokinetic behavior was independent of the frequency of reversals of gliding direction (klinokinesis) but did require both the Frz signal transduction system and S-motility. We propose that M. xanthus uses Frz-dependent, auto-orthokinetic behavior to facilitate the dispersal of cells under conditions where both cell density and nutrient levels are high.

Regulation of directed motility in Myxococcus xanthus

Myxococcus xanthus is a Gram-negative bacterium that exhibits a complex life cycle. During vegetative growth, cells move as large swarms. However, when starved, cells aggregate into fruiting bodies and sporulate. Both vegetative swarming and developmental aggregation require gliding motility, which involves the slow movement of cells on a solid surface in the absence of flagella. The frequency of cell reversals controls the direction of movement and is regulated by the frz genes, which encode the 'frizzy' signal-transduction proteins. These proteins contain domains which bear striking similarities to the major chemotaxis proteins of the enteric bacteria: CheA, CheY, CheW, CheR, CheB and Tar. However, significant differences exist between the Myxococcus Frz proteins and the enteric Che/MCP proteins. For example, the Frz system contains three CheY-like response-regulator domains: one is present on FrzE, which also contains a CheA-like domain, and two are present on FrzZ, which is a novel protein required for attractant, but not for repellent, responses. The identification of multiple CheY homologues in this system indicates a more complex regulatory pathway than that found in the enteric bacteria. While responses to repellent stimuli appear to follow the enteric paradigm, responses to attractants during vegetative swarming and development are more complex and may involve self-generated autoattractants. The Frz signal-transduction system regulates directed motility in M. xanthus and is essential for controlling both fruiting-body development and vegetative swarming.

Starvation-independent sporulation in Myxococcus xanthus involves the pathway for beta-lactamase induction and provides a mechanism for competitive cell survival

Myxococcus xanthus is a Gram-negative, soil-dwelling bacterium with a complex life cycle which includes fruiting body formation and sporulation in response to starvation. This developmental process is slow, requiring a minimum of 24-48 h, and requires cells to be at high cell density on a solid surface. It is known that, in the absence of starvation, vegetatively growing cell suspensions can form 'glycerol spores' when exposed to high levels of glycerol, usually 0.5 M. The cells differentiate from rods to resistant spheres rapidly (2-4 h) and synchronously. We have found that the chromosomally encoded beta-lactamase of M. xanthus can be induced by numerous beta-lactam antibiotics as well as by non-specific inducers including glycine and many D-amino acids. In addition, D-cycloserine, phosphomycin, and hen egg-white lysozyme also induce beta-lactamase in this bacterium. Unexpectedly, agents which induce beta-lactamase can induce 'glycerol spores'; all of the agents tested which induce glycerol spores (glycerol, DMSO, ethylene glycol) also induce beta-lactamase. During the induction of sporulation, beta-lactamase activity increases, reaching a peak during the morphological transition from rod-shaped cells to spherical spores. These spores are viable and resistant to many treatments which disrupt vegetatively growing rods but are not as resistant as fruiting body spores. The concomitant induction of beta-lactamase and starvation-independent sporulation suggests that these processes share a common signal-transduction pathway. These results also suggest that starvation-independent sporulation may be an adaptation of cells in order to resist agents that damage peptidoglycan structure and therefore threaten cell survival.

Identification and characterization of FrzZ, a novel response regulator necessary for swarming and fruiting-body formation in Myxococcus xanthus

The frz genes of Myxococcus xanthus constitute a signal-transduction pathway that processes chemotactic information in a manner analogous to that found in enteric bacteria. Ultimately, these genes regulate the frequency of individual cell reversal. We report here the identification of a novel component of this signal-transduction pathway, designated frzZ, which was discovered as an open reading frame located 5' to the frz operon but transcribed in the opposite orientation. The translational start site of frzZ is 170 base pairs from that of frzA.frzZ utilizes a promoter similar to the sigma 70 promoters of Escherichia coli, and encodes a 290-amino-acid soluble protein, FrzZ (M(r) 30,500). FrzZ contains two domains, both of which show strong homology to CheY and other members of the response-regulator family. Linking these domains is a 39-amino-acid region that is very rich in alanine and proline (38% Ala and 33% Pro). A frzZ null mutant showed abnormally low reversal rates when compared to the wild-type control and was unable to form fruiting bodies on starvation medium, but it did form 'frizzy' aggregates. In addition, the frzZ mutant was defective in swarming, particularly on soft agar (0.3% w/v). However, unlike most frz mutants, the frzZ mutant was able to respond to attractants and repellents in the spatial chemotaxis assay. The discovery of FrzZ demonstrates that the M. xanthus frz signal-transduction pathway utilizes multiple response-regulator (CheY-like) proteins.

Cell density regulates cellular reversal frequency in Myxococcus xanthus

Myxococcus xanthus is a Gram-negative bacterium that aggregates to form fruiting bodies when nutrients are limiting. Previous studies showed that the frz mutants that are defective in chemotaxis exhibited irregular and infrequent patterns of cellular reversal. In contrast, wild-type cells, when examined individually, reverse relatively frequently, about once every 6 min. It is not known how the change of reversal frequency effects cellular aggregation during fruiting body formation in M. xanthus. In this study, we stained cells with a tetrazolium dye so that we could track the reversal frequencies of single cells and cells in groups. We found that developmental cells in large groups reverse much less than cells in small groups or as single cells. This reduced cellular reversal frequency is related to the frz signal transduction system and correlated with the methylation of FrzCD (a methyl-accepting chemotaxis protein). Cells containing a mutation in the frz genes or in the genes required for social motility do not respond in this way. The reduction in cellular reversals as developmental cells accumulate in groups suggests a simple hypothesis for the aggregation of cells into discrete mounds during fruiting body formation. We also found that M. xanthus cells glide with equal frequency in the forward or reverse directions, indicating that cells do not contain a "head" or "tail."

Behavioral analysis of single cells of Myxococcus xanthus in response to prey cells of Escherichia coli

Myxococcus xanthus cells move over surfaces by gliding motility. The frz signal transduction system is used to control the reversal frequency, and thus the overall direction of movement of M. xanthus cells. We analyzed the behavior of wild-type and frz mutant cells in response to prey bacteria (Escherichia coli). Wild-type cells of M. xanthus did not respond to microcolonies of E. coli until they made physical contact. Cells which penetrated a colony remained in the colony until all of the prey cells were digested. Cells of frz mutants also penetrated E. coli microcolonies and digested some of the E. coli cells, but they invariably abandoned the microcolony leaving their food source behind. These observations illustrate the importance of the frz system of signal transduction for the feeding behavior of M. xanthus cells.