Oar, a 115-kilodalton membrane protein required for development of Myxococcus xanthus

Development versus predation: Transcriptomic changes during the lifecycle of Myxococcus xanthus

Myxococcus xanthus is a multicellular bacterium with a complex lifecycle. It is a soil-dwelling predator that preys on a wide variety of microorganisms by using a group and collaborative epibiotic strategy. In the absence of nutrients this myxobacterium enters in a unique developmental program by using sophisticated and complex regulatory systems where more than 1,400 genes are transcriptional regulated to guide the community to aggregate into macroscopic fruiting bodies filled of environmentally resistant myxospores. Herein, we analyze the predatosome of M. xanthus, that is, the transcriptomic changes that the predator undergoes when encounters a prey. This study has been carried out using as a prey Sinorhizobium meliloti, a nitrogen fixing bacteria very important for the fertility of soils. The transcriptional changes include upregulation of genes that help the cells to detect, kill, lyse, and consume the prey, but also downregulation of genes not required for the predatory process. Our results have shown that, as expected, many genes encoding hydrolytic enzymes and enzymes involved in biosynthesis of secondary metabolites increase their expression levels. Moreover, it has been found that the predator modifies its lipid composition and overproduces siderophores to take up iron. Comparison with developmental transcriptome reveals that M. xanthus downregulates the expression of a significant number of genes coding for regulatory elements, many of which have been demonstrated to be key elements during development. This study shows for the first time a global view of the M. xanthus lifecycle from a transcriptome perspective.

A novel outer membrane β-1,6-glucanase is deployed in the predation of fungi by myxobacteria

Myxobacterial predation on bacteria has been investigated for several decades. However, their predation on fungi has received less attention. Here, we show that a novel outer membrane β-1,6-glucanase GluM from Corallococcus sp. strain EGB is essential for initial sensing and efficient decomposition of fungi during predation. GluM belongs to an unstudied family of outer membrane β-barrel proteins with potent specific activity up to 24,000 U/mg, whose homologs extensively exist in myxobacteria. GluM was able to digest fungal cell walls efficiently and restrict Magnaporthe oryzae infection of rice plants. Genetic complementation with gluM restored the fungal predation ability of Myxococcus xanthus CL1001, which was abolished by the disruption of gluM homolog oar. The inability to prey on fungi with cell walls that lack β-1,6-glucans indicates that β-1,6-glucans are targeted by GluM. Our results demonstrate that GluM confers myxobacteria with the ability to feed on fungi, and provide new insights for understanding predator-prey interactions. Considering the attack mode of GluM, we suggest that β-1,6-glucan is a promising target for the development of novel broad-spectrum antifungal agents.

A TonB-dependent transporter is required for secretion of protease PopC across the bacterial outer membrane

TonB-dependent transporters (TBDTs) are ubiquitous outer membrane β-barrel proteins that import nutrients and bacteriocins across the outer membrane in a proton motive force-dependent manner, by directly connecting to the ExbB/ExbD/TonB system in the inner membrane. Here, we show that the TBDT Oar in Myxococcus xanthus is required for secretion of a protein, protease PopC, to the extracellular milieu. PopC accumulates in the periplasm before secretion across the outer membrane, and the proton motive force has a role in secretion to the extracellular milieu. Reconstitution experiments in Escherichia coli demonstrate that secretion of PopC across the outer membrane not only depends on Oar but also on the ExbB/ExbD/TonB system. Our results indicate that TBDTs and the ExbB/ExbD/TonB system may have roles not only in import processes but also in secretion of proteins.

TonB-dependent transporters (TBDTs) are outer membrane proteins that import nutrients and bacteriocins in bacteria. Here, Gómez-Santos et al. show that a TBDT is required for secretion of a protease in Myxococcus xanthus, suggesting that some TBDTs may be involved in protein secretion.

Dissection of the sensor domain of the copper-responsive histidine kinase CorS from Myxococcus xanthus

Myxococcus xanthus CorSR is a two-component system responsible for maintaining the response of this bacterium to copper. In the presence of this metal it upregulates, among others, the genes encoding the multicopper oxidase CuoA and the P1B -ATPase CopA. Dissection of the periplasmic sensor domain of the histidine kinase CorS by the analysis of a series of in-frame deletion mutants generated in this portion of the protein has revealed that copper sensing requires a region of 28 residues in the N terminus and another region of nine residues in the C terminus. Point mutations at His34, His38 and His171 demonstrate that they are essential for the ability of CorS to sense copper. Furthermore, the use of a bacterial two-hybrid system has revealed dimerization between monomers of CorS even in the absence of any metal, and that copper enhances this interaction. When dimerization was tested with proteins mutated at the three essential His residues, it was observed that these proteins maintain the intrinsic dimerization ability in the absence of metal. In contrast to the wild-type protein, copper did not strengthen the interaction, corroborating that copper binding to the three His residues of CorS is required for enhancing dimerization and transmitting the signal.

In depth analysis of the mechanism of action of metal-dependent sigma factors: characterization of CorE2 from Myxococcus xanthus

Extracytoplasmic function sigma factors represent the third pillar of signal-transduction mechanisms in bacteria. The variety of stimuli they recognize and mechanisms of action they use have allowed their classification into more than 50 groups. We have characterized CorE2 from Myxococcus xanthus, which belongs to group ECF44 and upregulates the expression of two genes when it is activated by cadmium and zinc. Sigma factors of this group contain a Cys-rich domain (CRD) at the C terminus which is essential for detecting metals. Point mutations at the six Cys residues of the CRD have revealed the contribution of each residue to CorE2 activity. Some of them are essential, while others are either dispensable or their mutations only slightly affect the activity of the protein. However, importantly, mutation of Cys174 completely shifts the specificity of CorE2 from cadmium to copper, indicating that the Cys arrangement of the CRD determines the metal specificity. Moreover, the conserved CxC motif located between the σ2 domain and the σ4.2 region has also been found to be essential for activity. The results presented here contribute to our understanding of the mechanism of action of metal-dependent sigma factors and help to define new common features of the members of this group of regulators.

Contact- and Protein Transfer-Dependent Stimulation of Assembly of the Gliding Motility Machinery in Myxococcus xanthus

Bacteria engage in contact-dependent activities to coordinate cellular activities that aid their survival. Cells of Myxococcus xanthus move over surfaces by means of type IV pili and gliding motility. Upon direct contact, cells physically exchange outer membrane (OM) lipoproteins, and this transfer can rescue motility in mutants lacking lipoproteins required for motility. The mechanism of gliding motility and its stimulation by transferred OM lipoproteins remain poorly characterized. We investigated the function of CglC, GltB, GltA and GltC, all of which are required for gliding. We demonstrate that CglC is an OM lipoprotein, GltB and GltA are integral OM β-barrel proteins, and GltC is a soluble periplasmic protein. GltB and GltA are mutually stabilizing, and both are required to stabilize GltC, whereas CglC accumulate independently of GltB, GltA and GltC. Consistently, purified GltB, GltA and GltC proteins interact in all pair-wise combinations. Using active fluorescently-tagged fusion proteins, we demonstrate that GltB, GltA and GltC are integral components of the gliding motility complex. Incorporation of GltB and GltA into this complex depends on CglC and GltC as well as on the cytoplasmic AglZ protein and the inner membrane protein AglQ, both of which are components of the gliding motility complex. Conversely, incorporation of AglZ and AglQ into the gliding motility complex depends on CglC, GltB, GltA and GltC. Remarkably, physical transfer of the OM lipoprotein CglC to a ΔcglC recipient stimulates assembly of the gliding motility complex in the recipient likely by facilitating the OM integration of GltB and GltA. These data provide evidence that the gliding motility complex in M. xanthus includes OM proteins and suggest that this complex extends from the cytoplasm across the cell envelope to the OM. These data add assembly of gliding motility complexes in M. xanthus to the growing list of contact-dependent activities in bacteria.

Motility facilitates a wide variety of processes such as virulence, biofilm formation and development in bacteria. Bacteria have evolved at least three mechanisms for motility on surfaces: swarming motility, twitching motility and gliding motility. Mechanistically, gliding motility is poorly understood. Here, we focused on four proteins in Myxococcus xanthus that are essential for gliding. We show that CglC is an outer membrane (OM) lipoprotein, GltB and GltA are integral OM β-barrel proteins, and GltC is a soluble periplasmic protein. GltB, GltA and GltC are components of the gliding motility complex, and CglC likely stimulates the integration of GltB and GltA into the OM. Moreover, CglC, in a cell-cell contact-dependent manner, can be transferred from a cglC⁺ donor to a ΔcglC mutant leading to stimulation of gliding motility in the recipient. We show that upon physical transfer of CglC, CglC stimulates the assembly of the gliding motility complex in the recipient. The data presented here adds to the growing list of cell-cell contact-dependent activities in bacteria by demonstrating that gliding motility can be stimulated in a contact-dependent manner by transfer of a protein that stimulates assembly of the gliding motility complexes.

Rice proteomics: a model system for crop improvement and food security

Rice proteomics has progressed at a tremendous pace since the year 2000, and that has resulted in establishing and understanding the proteomes of tissues, organs, and organelles under both normal and abnormal (adverse) environmental conditions. Established proteomes have also helped in re-annotating the rice genome and revealing the new role of previously known proteins. The progress of rice proteomics had recognized it as the corner/stepping stone for at least cereal crops. Rice proteomics remains a model system for crops as per its exemplary proteomics research. Proteomics-based discoveries in rice are likely to be translated in improving crop plants and vice versa against ever-changing environmental factors. This review comprehensively covers rice proteomics studies from August 2010 to July 2013, with major focus on rice responses to diverse abiotic (drought, salt, oxidative, temperature, nutrient, hormone, metal ions, UV radiation, and ozone) as well as various biotic stresses, especially rice-pathogen interactions. The differentially regulated proteins in response to various abiotic stresses in different tissues have also been summarized, indicating key metabolic and regulatory pathways. We envision a significant role of rice proteomics in addressing the global ground level problem of food security, to meet the demands of the human population which is expected to reach six to nine billion by 2040.

Secretome analysis of the rice bacterium Xanthomonas oryzae (Xoo) using in vitro and in planta systems

Xanthomonas oryzae pv. oryzae (Xoo) causes bacterial blight disease in rice, and that severely affects yield loss (upto 50%) of total rice production. Here, we report a proteomics investigation of Xoo (compatible race K3)-secreted proteins, isolated from its in vitro culture and in planta infected rice leaves. 2DE coupled with MALDI-TOF-MS and/or nLC-ESI-MS/MS approaches identified 139 protein spots (out of 153 differential spots), encoding 109 unique proteins. Identified proteins belonged to multiple biological and molecular functions. Metabolic and nutrient uptake proteins were common up to both in vitro and in planta secretomes. However, pathogenicity, protease/peptidase, and host defense-related proteins were highly or specifically expressed during in planta infection. A good correlation was observed between protein and transcript abundances for nine proteins secreted in planta as per semiquantitative RT-PCR analysis. Transgenic rice leaf sheath (carrying PBZ1 promoter::GFP cell death reporter), when used to express a few of the identified secretory proteins, showed a direct activation of cell death signaling, suggesting their involvement in pathogenicity related with secretion effectors. This work furthers our understanding of rice bacterial blight disease, and serves as a resource for possible translation in generating disease resistant rice plants for improved seed yield.

Two systems for conditional gene expression in Myxococcus xanthus inducible by isopropyl-β-D-thiogalactopyranoside or vanillate

Conditional expression of a gene is a powerful tool to study its function and is typically achieved by placing the gene under the control of an inducible promoter. There is, however, a dearth of such inducible systems in Myxococcus xanthus, a well-studied prokaryotic model for multicellular development, cell differentiation, motility, and light response and a promising source of secondary metabolites. The few available systems have limitations, and exogenously based ones are unavailable. Here, we describe two new, versatile inducible systems for conditional expression of genes in M. xanthus. One employs isopropyl-β-d-thiogalactopyranoside (IPTG) as an inducer and is inspired by those successfully applied in some other bacteria. The other requires vanillate as an inducer and is based on the system developed originally for Caulobacter crescentus and recently adapted for mammalian cells. Both systems are robust, with essentially no expression in the absence of an inducer. Depending on the inducer and the amounts added, expression levels can be modulated such that either system can conditionally express genes, including ones that are essential and are required at high levels such as ftsZ. The two systems operate during vegetative growth as well as during M. xanthus development. Moreover, they can be used to simultaneously induce expression of distinct genes within the same cell. The conditional expression systems we describe substantially expand the genetic tool kit available for studying M. xanthus gene function and cellular biology.

Comprehensive set of integrative plasmid vectors for copper-inducible gene expression in Myxococcus xanthus

Myxococcus xanthus is widely used as a model system for studying gliding motility, multicellular development, and cellular differentiation. Moreover, M. xanthus is a rich source of novel secondary metabolites. The analysis of these processes has been hampered by the limited set of tools for inducible gene expression. Here we report the construction of a set of plasmid vectors to allow copper-inducible gene expression in M. xanthus. Analysis of the effect of copper on strain DK1622 revealed that copper concentrations of up to 500 μM during growth and 60 μM during development do not affect physiological processes such as cell viability, motility, or aggregation into fruiting bodies. Of the copper-responsive promoters in M. xanthus reported so far, the multicopper oxidase cuoA promoter was used to construct expression vectors, because no basal expression is observed in the absence of copper and induction linearly depends on the copper concentration in the culture medium. Four different plasmid vectors have been constructed, with different marker selection genes and sites of integration in the M. xanthus chromosome. The vectors have been tested and gene expression quantified using the lacZ gene. Moreover, we demonstrate the functional complementation of the motility defect caused by lack of PilB by the copper-induced expression of the pilB gene. These versatile vectors are likely to deepen our understanding of the biology of M. xanthus and may also have biotechnological applications.

Identification and localization of Myxococcus xanthus porins and lipoproteins

Myxococcus xanthus DK1622 contains inner (IM) and outer membranes (OM) separated by a peptidoglycan layer. Integral membrane, β-barrel proteins are found exclusively in the OM where they form pores allowing the passage of nutrients, waste products and signals. One porin, Oar, is required for intercellular communication of the C-signal. An oar mutant produces CsgA but is unable to ripple or stimulate csgA mutants to develop suggesting that it is the channel for C-signaling. Six prediction programs were evaluated for their ability to identify β-barrel proteins. No program was reliable unless the predicted proteins were first parsed using Signal P, Lipo P and TMHMM, after which TMBETA-SVM and TMBETADISC-RBF identified β-barrel proteins most accurately. 228 β-barrel proteins were predicted from among 7331 protein coding regions, representing 3.1% of total genes. Sucrose density gradients were used to separate vegetative cell IM and OM fractions, and LC-MS/MS of OM proteins identified 54 β-barrel proteins. Another class of membrane proteins, the lipoproteins, are anchored in the membrane via a lipid moiety at the N-terminus. 44 OM proteins identified by LC-MS/MS were predicted lipoproteins. Lipoproteins are distributed between the IM, OM and ECM according to an N-terminal sorting sequence that varies among species. Sequence analysis revealed conservation of alanine at the +7 position of mature ECM lipoproteins, lysine at the +2 position of IM lipoproteins, and no noticable conservation within the OM lipoproteins. Site directed mutagenesis and immuno transmission electron microscopy showed that alanine at the +7 position is essential for sorting of the lipoprotein FibA into the ECM. FibA appears at normal levels in the ECM even when a +2 lysine is added to the signal sequence. These results suggest that ECM proteins have a unique method of secretion. It is now possible to target lipoproteins to specific IM, OM and ECM locations by manipulating the amino acid sequence near the +1 cysteine processing site.

CorE from Myxococcus xanthus is a copper-dependent RNA polymerase sigma factor

The dual toxicity/essentiality of copper forces cells to maintain a tightly regulated homeostasis for this metal in all living organisms, from bacteria to humans. Consequently, many genes have previously been reported to participate in copper detoxification in bacteria. Myxococcus xanthus, a prokaryote, encodes many proteins involved in copper homeostasis that are differentially regulated by this metal. A σ factor of the ECF (extracytoplasmic function) family, CorE, has been found to regulate the expression of the multicopper oxidase cuoB, the P1B-type ATPases copA and copB, and a gene encoding a protein with a heavy-metal-associated domain. Characterization of CorE has revealed that it requires copper to bind DNA in vitro. Genes regulated by CorE exhibit a characteristic expression profile, with a peak at 2 h after copper addition. Expression rapidly decreases thereafter to basal levels, although the metal is still present in the medium, indicating that the activity of CorE is modulated by a process of activation and inactivation. The use of monovalent and divalent metals to mimic Cu(I) and Cu(II), respectively, and of additives that favor the formation of the two redox states of this metal, has revealed that CorE is activated by Cu(II) and inactivated by Cu(I). The activation/inactivation properties of CorE reside in a Cys-rich domain located at the C terminus of the protein. Point mutations at these residues have allowed the identification of several Cys involved in the activation and inactivation of CorE. Based on these data, along with comparative genomic studies, a new group of ECF σ factors is proposed, which not only clearly differs mechanistically from the other σ factors so far characterized, but also from other metal regulators.

Copper exerts a dual effect on living organisms. It is essential for life, but an excess provokes cell damage, forcing cells to maintain a regulated homeostasis for this metal. These two antagonistic biological effects of copper are clearly illustrated by two human genetic disorders, Menkes syndrome and Wilson disease, caused by deficiency or accumulation of this metal, respectively. Myxococcus xanthus, a soil-dwelling bacterium, also has to cope with changes in copper concentration in its environment. The large genome of this myxobacterium encodes many genes involved in copper homeostasis, all of which are differentially regulated, indicating that many regulators participate in copper homeostasis in this prokaryote. Here, we identify one of these regulators (CorE), which belongs to the family of the extracytoplasmic function (ECF) σ factors. We demonstrate that CorE represents a novel group of ECF σ factors and of metal regulators, because its activity is modulated by the redox state of copper. This ability resides in a Cys-rich domain, which has also been found in other σ factors of different bacterial phyla. Therefore, we propose that CorE is the first member of a mechanistically new group of ECF σ factors.

BacM, an N-terminally processed bactofilin of Myxococcus xanthus, is crucial for proper cell shape

Bactofilins are fibre-forming bacterial cytoskeletal proteins. Here, we report the structural and biochemical characterization of MXAN_7475 (BacM), one of the four bactofilins of Myxococcus xanthus. Absence of BacM leads to a characteristic 'crooked' cell morphology and an increased sensitivity to antibiotics targeting cell wall biosynthesis. The absence of the other three bactofilins MXAN_4637-4635 (BacN-P) has no obvious phenotype. In M. xanthus, BacM exists as a 150-amino-acid full-length version and as a version cleaved before Ser28. In the cell, native BacM forms 3 nm wide fibres, which assemble into bundles forming helix-like cytoplasmic cables throughout the cell, and in a subset of cells additionally a polarly arranged lateral rod-like structure. Isolated fibres consist almost completely of the N-terminally truncated version, suggesting that the proteolytic cleavage occurs before or during fibre formation. Fusion of BacM to mCherry perturbs BacM function and cellular fibre arrangement, resulting for example in the formation of one prominent polar corkscrew-like structure per cell. Immunofluorescence staining of BacM and MreB shows that their cellular distributions are not matching. Taken together, these data suggest that rod-shaped bacteria like M. xanthus use bactofilin fibres to achieve and maintain their characteristic cell morphology and cell wall stability.

Profiling the outer membrane proteome during growth and development of the social bacterium Myxococcus xanthus by selective biotinylation and analyses of outer membrane vesicles

Social behavior in the bacterium Myxococcus xanthus relies on contact-dependent activities involving cell-cell and cell-substratum interactions. To identify outer membrane proteins that have a role in these activities, we profiled the outer membrane proteome of growing and starving cells using two strategies. First, outer membrane proteins were enriched by biotinylation of intact cells using the reagent NHS (N-hydroxysuccinimide)-PEO(12) (polyethylene oxide)-biotin with subsequent membrane solubilization and affinity chromatography. Second, the proteome of outer membrane vesicles (OMV) was determined. Comparisons of detected proteins show that these methods have different detection profiles and together provide a comprehensive view of the outer membrane proteome. From 362 proteins identified, 274 (76%) were cell envelope proteins including 64 integral outer membrane proteins and 85 lipoproteins. The majority of these proteins were of unknown function. Among integral outer membrane proteins with homologues of known function, TonB-dependent transporters comprise the largest group. Our data suggest novel functions for these transporters. Among lipoproteins with homologues of known function, proteins with hydrolytic functions comprise the largest group. The luminal load of OMV was enriched for proteins with hydrolytic functions. Our data suggest that OMV have functions in predation and possibly in transfer of intercellular signaling molecules between cells.

Nla18, a key regulatory protein required for normal growth and development of Myxococcus xanthus

NtrC-like activators regulate the transcription of a wide variety of adaptive genes in bacteria. Previously, we demonstrated that a mutation in the ntrC-like activator gene nla18 causes defects in fruiting body development in Myxococcus xanthus. In this report, we describe the effect that nla18 inactivation has on gene expression patterns during development and vegetative growth. Gene expression in nla18 mutant cells is altered in the early stages of fruiting body development. Furthermore, nla18 mutant cells are defective for two of the earliest events in development, production of the intracellular starvation signal ppGpp and production of A-signal. Taken together, these results indicate that the developmental program in nla18 mutant cells goes awry very early. Inactivation of nla18 also causes a dramatic decrease in the vegetative growth rate of M. xanthus cells. DNA microarray analysis revealed that the vegetative expression patterns of more than 700 genes are altered in nla18 mutant cells. Genes coding for putative membrane and membrane-associated proteins are among the largest classes of genes whose expression is altered by nla18 inactivation. This result is supported by our findings that the profiles of membrane proteins isolated from vegetative nla18 mutant and wild-type cells are noticeably different. In addition to genes that code for putative membrane proteins, nla18 inactivation affects the expression of many genes that are likely to be important for protein synthesis and gene regulation. Our data are consistent with a model in which Nla18 controls vegetative growth and development by activating the expression of genes involved in gene regulation, translation, and membrane structure.

TonB-dependent trans-envelope signalling: the exception or the rule?

TonB-dependent regulatory systems consist of six components, a specialized outer membrane-localized TonB-dependent receptor (TonB-dependent transducer) that interacts with its energizing TonB-ExbBD protein complex, a cytoplasmic membrane-localized anti-sigma factor and an extracytoplasmic function (ECF)-subfamily sigma factor. This sophisticated complex senses signals from outside the bacterial cell and transmits them via two membranes into the cytoplasm, leading to transcriptional activation of target genes. TonB-dependent transducers might be important for pathogenicity; therefore, a survey of their presence in bacteria was aimed. Genome-wide analyses indicate that these systems are commonly found in several environmental bacteria but are only seldom present in human and animal pathogens. Most systems have the same tandem organization of the corresponding genes: sigma factor-anti-sigma factor-TonB-dependent transducer. In the course of these analyses, a novel protein domain of unknown function was identified and four different modular structures of TonB-dependent receptors were detected.

Proteomic analysis of the Caulobacter crescentus stalk indicates competence for nutrient uptake

Caulobacter crescentus, a Gram-negative alpha-purple proteobacterium, is an oligotroph that lives in aquatic environments dilute in nutrients. This bacterium divides asymmetrically. Part of this asymmetric cell division involves the formation of a prosthecum at one pole, referred to as the stalk, which replaces the flagellum of the motile swarmer cell. Little is known about the synthesis or function of the stalk. The stalk is an extension of the cell membranes and peptidoglycan layer, and stalk elongation is stimulated by phosphate starvation. In this study, we have taken advantage of two-dimensional gel (2D gel) electro-phoresis as well as the fully sequenced genome of Caulobacter to study the proteome of the stalk. We modified a stalk-shedding mutant strain of Caulobacter crescentus to increase the yield of stalk material shed and performed 2D gel electrophoresis of purified stalks and cellular fractions. Comparison of the stalk 2D gel with the 2D gels of cell membrane and soluble fractions showed that the stalk is mostly free of cytoplasmic proteins and has a profile very similar to that of the cell membrane. Of the 172 proteins on a stalk 2D gel, we report the identification of 64 spots, corresponding to 39 different proteins present in the stalk of Caulobacter. The identifications include several TonB-dependent receptors, two OmpA family proteins, a dipeptidase, GlpQ, two alkaline phosphatases, 3-phytase, a putative TolC protein and 11 proteins of unknown function. These identifications are consistent with the hypothesis that the stalk plays a role in nutrient uptake.

The FruA signal transduction protein provides a checkpoint for the temporal co-ordination of intercellular signals in Myxococcus xanthus development

During fruiting body morphogenesis in Myxococcus xanthus, the intercellular C-signal induces aggregation, sporulation and developmental gene expression. To understand how a single signal system may induce temporally separated processes, we have focused on the class II gene, which codes for an essential component in the C-signal transduction pathway. We report that class II is identical to fruA and codes for a DNA binding response regulator. Transcription of fruA is developmentally regulated and depends on the early acting intercellular A- and E-signals. However, fruA transcription is independent of C-signal. Rather, genetic evidence suggests that C-signal controls FruA activity post-translationally. Genetic evidence strongly indicates that FruA is activated by phosphorylation. We propose that C-signalling results in the phosphorylation of FruA, thus activating FruA to interact with downstream targets. In the motility branch of the C-signalling pathway, FruA interacts with the Frz motility system; in the sporulation branch, we show that FruA is required for transcription of the sporulation locus devRS. On the basis of the two levels of control of FruA activity, we propose that FruA serves as a control point for the temporal co-ordination of intercellular signals during M. xanthus development.

MlpA, a lipoprotein required for normal development of Myxococcus xanthus

The mlpA gene encoding a 236-residue polypeptide has been identified immediately downstream of the oar gene of Myxococcus xanthus (M. Martinez-Canamero, J. Munoz-Dorado, E. Farez-Vidal, M. Inouye, and S. Inouye, J. Bacteriol. 175:4756-4763, 1993). The amino-terminal 21 residues of MlpA encode a typical prokaryotic signal sequence with a putative lipoprotein cleavage site. When expressed in Escherichia coli in the presence of [2-3H]glycerol, 3H-labeled MlpA had a molecular mass of 33 kDa and was found to be associated with the membrane fraction. Globomycin, an inhibitor of signal peptidase II, caused a shift in the mobility of E. coli-expressed MlpA to 35 kDa. Subsequently, a mlpA disruption strain (oar+) was constructed and found to have delayed fruiting body formation (by approximately 36 h), with significantly larger fruiting bodies being produced compared with those of the wild-type strain. Nevertheless, spore yields for the two strains were identical after 120 h of development. These data indicate that MlpA, the lipoprotein identified in M. xanthus, is required for normal fruiting body formation.