In situ transposon replacement and isolation of a spontaneous tandem genetic duplication

Genetic manipulation and tools in myxobacteria for the exploitation of secondary metabolism

Myxobacteria are famous for their capacity for social behavior and natural product biosynthesis. The unique sociality of myxobacteria is not only an intriguing scientific topic but also the main limiting factor for their manipulation. After more than half a century of research, a series of genetic techniques for myxobacteria have been developed, rendering these mysterious bacteria manipulable. Here, we review the advances in genetic manipulation of myxobacteria, with a particular focus on the exploitation of secondary metabolism. We emphasize the necessity and urgency of constructing the myxobacterial chassis for synthetic biology research and the exploitation of untapped secondary metabolism.

AmyM, a Novel Maltohexaose-Forming α-Amylase from Corallococcus sp. strain EGB

A novel α-amylase, AmyM, was purified from the culture supernatant of Corallococcus sp. strain EGB. AmyM is a maltohexaose-forming exoamylase with an apparent molecular mass of 43 kDa. Based on the results of matrix-assisted laser desorption ionization-time of flight mass spectrometry and peptide mass fingerprinting of AmyM and by comparison to the genome sequence of Corallococcus coralloides DSM 2259, the AmyM gene was identified and cloned into Escherichia coli. amyM encodes a secretory amylase with a predicted signal peptide of 23 amino acid residues, which showed no significant identity with known and functionally verified amylases. amyM was expressed in E. coli BL21(DE3) cells with a hexahistidine tag. The signal peptide efficiently induced the secretion of mature AmyM in E. coli. Recombinant AmyM (rAmyM) was purified by Ni-nitrilotriacetic acid (NTA) affinity chromatography, with a specific activity of up to 14,000 U/mg. rAmyM was optimally active at 50°C in Tris-HCl buffer (50 mM; pH 7.0) and stable at temperatures of <50°C. rAmyM was stable over a wide range of pH values (from pH 5.0 to 10.0) and highly tolerant to high concentrations of salts, detergents, and various organic solvents. Its activity toward starch was independent of calcium ions. The Km and Vmax of recombinant AmyM for soluble starch were 6.61 mg ml(-1) and 44,301.5 μmol min(-1) mg(-1), respectively. End product analysis showed that maltohexaose accounted for 59.4% of the maltooligosaccharides produced. These characteristics indicate that AmyM has great potential in industrial applications.

Light induction of gene expression in Myxococcus xanthus

The synthesis of carotenoids by Myxococcus xanthus requires illumination with blue light. Mutations at two loci (carA and carR) remove the blue-light requirement and cause constitutive production of carotenoids. Mutations at a different locus (carB) prevent carotenogenesis in both wild-type and constitutive mutant strains. We describe here three independent car mutations induced by insertion of Tn5 lac, a transposon that carries a transcriptional probe for exogenous promoters. All three transposon insertions block carotenogenesis even in constitutive mutant strains. One insertion is in a previously unknown car gene and the other two are in the carB locus. One of the carB insertions expresses beta-galactosidase at very low levels in the dark but is strongly activated by light. When this Tn5 lac insertion is introduced in carA or carR mutants it expresses beta-galactosidase in dark- as well as light-grown cells. We conclude that carotenogenesis in M. xanthus is activated at the level of transcription by a light-induced mechanism in which the carA and the carR loci (or their gene products) take part. The potential usefulness of M. xanthus as a simple and sensitive tool for studies in photobiology is discussed.

Generalized transduction

Transduction is the process in which bacterial DNA is transferred from one bacterial cell to another by means of a phage particle. There are two types of transduction, generalized transduction and specialized transduction. In this chapter two of the best-studied systems - Escherichia coli-phage P1, and Salmonella enterica-phage P22 - are discussed from theoretical and practical perspectives.

Vitamin B12 partners the CarH repressor to downregulate a photoinducible promoter in Myxococcus xanthus

A light-inducible promoter, P(B), drives expression of the carB operon in Myxococcus xanthus. Repressed by CarA in the dark, P(B) is activated when CarS, produced in the light, sequesters CarA to prevent operator-CarA binding. The MerR-type, N-terminal domain of CarA, which mediates interactions with both operator and CarS, is linked to a C-terminal oligomerization module with a predicted cobalamin-binding motif. Here, we show that although CarA does bind vitamin B12, mutating the motif involved has no effect on its ability to repress P(B). Intriguingly, P(B) could be repressed in the dark even with no CarA, so long as B12 and an intact CarA operator were present. We have discovered that this effect of B12 depends on the gene immediately downstream of carA. Its product, CarH, also consists of a MerR-type, N-terminal domain that specifically recognizes the CarA operator and CarS, linked to a predicted B12-binding C-terminal oligomerization module. The B12-mediated repression of P(B) in the dark is relieved by deleting carH, by mutating the DNA- or B12-binding residues of CarH, or by illumination. Our findings unveil parallel regulatory circuits that control a light-inducible promoter using a transcriptional factor repertoire that includes a paralogous gene pair and vitamin B12.

A microbial genetic journey

Fortunately, I began research in 1950 when the basic concepts of microbial genetics could be explored experimentally. I began with bacteriophage lambda and tried to establish the colinearity of its linkage map with its DNA molecule. My students and I worked out the regulation of lambda repressor synthesis for the establishment and maintenance of lysogeny. We also investigated the proteins responsible for assembly of the phage head. Using cell extracts, we discovered how to package DNA inside the head in vitro. Around 1972, I began to use molecular genetics to understand the developmental biology of Myxococcus xanthus. In particular, I wanted to learn how myxococcus builds its multicellular fruiting body within which it differentiates spores. We identified two cell-to-cell signals used to coordinate development. We have elucidated, in part, the signal transduction pathway for C-signal that directs the morphogenesis of a fruiting body.

The high-mobility group A-type protein CarD of the bacterium Myxococcus xanthus as a transcription factor for several distinct vegetative genes

CarD is the only reported prokaryotic protein showing structural and functional features typical of eukaryotic high-mobility group A transcription factors. In prokaryotes, proteins similar to CarD appear to be confined primarily to myxobacteria. In Myxococcus xanthus, CarD has been previously shown to act as a positive element in two different regulatory networks: one for light-induced synthesis of carotenoids and the other for starvation-induced fruiting body formation. We have now tested the effect of a loss-of-function mutation in the carD gene (carD1) on the expression of a random collection of lacZ-tagged genes, which are normally expressed in the dark during vegetative growth in rich medium. Our results indicate that CarD plays a significant role in the transcriptional regulation of various indicated genes. The carD1 mutation downregulates some genes and upregulates others. Also reported here is the isolation of several mutations that suppress the strong effect of carD1 on the expression of a particular vegetative gene. One of them (sud-2) also suppresses the effect of carD1 on other vegetative genes and on fruiting-body formation. Thus, CarD and the sud-2 gene product appear to participate in a single mechanism, which underlies various apparently diverse regulatory phenomena ascribed to CarD.

The N terminus of Myxococcus xanthus CarA repressor is an autonomously folding domain that mediates physical and functional interactions with both operator DNA and antirepressor protein

Expression of the Myxococcus xanthus carB operon, which encodes the majority of the enzymes involved in light-induced carotenogenesis, is down-regulated in the dark by the CarA repressor binding to its bipartite operator. CarS, produced on illumination, relieves repression of carB by physically interacting with CarA to dis-mantle CarA-DNA complexes. Here, we demonstrate that the N- and C-terminal portions of CarA are organized as distinct structural and functional domains. Specifically, we show that the 78 N-terminal residues of CarA, CarA(Nter), form a monomeric, highly helical, autonomously folding unit with significant structural stability. Significantly, CarA(Nter) houses both the operator and CarS binding specificity determinants of CarA. CarA(Nter) binds operator with a lower affinity than whole CarA, and the CarA(Nter)-CarS complex has a 1:1 stoichiometry. In vitro, sufficiently high concentrations of CarA(Nter) block M. xanthus RNA polymerase-promoter binding, and this is relieved by CarS. In vivo, substitution of the gene carA by that for CarA(Nter) results in constitutive expression of carB just as in a carA-deleted background. However, re-engineering the latter strain to overexpress CarA(Nter) restores repression of carB. Thus, the 78-residue N-terminal portion of CarA is an autonomously folded, dual function domain that orchestrates specific DNA-protein and protein-protein interactions and, when overexpressed, can be functionally competent in vivo.

Role of two novel two-component regulatory systems in development and phosphatase expression in Myxococcus xanthus

We have cloned a two-component regulatory system (phoR2-phoP2) of Myxococcus xanthus while searching for genes that encode proteins with phosphatase activity, where phoR2 encodes the histidine kinase and phoP2 encodes the response regulator. A second system, phoR3-phoP3, was identified and isolated by using phoP2 as a probe. These two systems are quite similar, sharing identities along the full-length proteins of 52% on the histidine kinases and 64% on the response regulators. The predicted structures of both kinases suggest that they are anchored to the membrane, with the sensor domains being located in the periplasmic space and the kinase domains in the cytoplasm. The response regulators (PhoP2 and PhoP3) exhibit a helix-loop-helix motif typical of DNA-binding proteins in the effector domains located in the C-terminal region. Studies on two single-deletion mutants and one double-deletion mutant have revealed that these systems are involved in development. Mutant fruiting bodies are not well packed, originating loose and flat aggregates where some myxospores do not reshape properly, and they remain as elongated cells. These systems are also involved in the expression of Mg-independent acid and neutral phosphatases, which are expressed during development. The neutral phosphatase gene is especially dependent on PhoP3. Neither PhoP2 nor PhoP3 regulates the expression of alkaline phosphatases and the pph1 gene.

A novel regulatory gene for light-induced carotenoid synthesis in the bacterium Myxococcus xanthus

Myxococcus xanthus cells respond to blue light by producing carotenoids. Light triggers a network of regulatory actions that lead to the transcriptional activation of the carotenoid genes. By screening the colour phenotype of a collection of Tn5-lac insertion mutants, we have isolated a new mutant devoid of carotenoid synthesis. We map the transposon insertion, which co-segregates with the mutant phenotype, to a previously unknown gene designated here as carF. An in frame deletion within carF causes the same phenotype as the Tn5-lac insertion. The carF deletion prevents the activation of the normally light-inducible genes, without affecting the expression of any of the regulatory genes known to be expressed in a light-independent manner. Until now, the switch that sets off the regulatory cascade had been identified with light-driven inactivation of protein CarR, an antisigma factor. The exact mechanism of this inactivation has remained elusive. We show by epistatic analysis that the carF gene product participates in the light-dependent inactivation of CarR. The predicted CarF amino acid sequence reveals no known prokaryotic homologues. On the other hand, CarF is remarkably similar to Kua, a family of proteins of unknown function that is widely distributed among eukaryotes.

Characterization of bcsA mutations that bypass two distinct signaling requirements for Myxococcus xanthus development

The BsgA protease is required for starvation-induced development in Myxococcus xanthus. Bypass suppressors of a bsgA mutant were isolated to identify genes that may encode additional components of BsgA protease-dependent regulation of development. Strain M951 was isolated following Tn5 mutagenesis of a bsgA mutant and was capable of forming fruiting bodies and viable spores in the absence of the BsgA protease. The Tn5Omega951 insertion was localized to a gene, bcsA, that encodes a protein that has significant amino acid similarity to a group of recently described flavin-containing monooxygenases involved in styrene catabolism. Mutations in bcsA bypassed the developmental requirements for both extracellular B and C signaling but did not bypass the requirement for A signaling. Bypass of the B-signaling requirement by the bcsA mutation was accompanied by restored expression of a subset of developmentally induced lacZ fusions to the BsgA protease-deficient strain. bcsA mutant cells developed considerably faster than wild-type cells at low cell density and altered transcriptional levels of a developmentally induced, cell-density-regulated gene (Omega4427), suggesting that the bcsA gene product may normally act to inhibit development in a cell-density-regulated fashion. Bypass of the requirements for both B and C signaling by bcsA mutations suggests a possible link between these two genetically, biochemically, and temporally distinct signaling requirements.

Heterologous expression of epothilone biosynthetic genes in Myxococcus xanthus

Epothilones are potential anticancer drugs that stabilize microtubules in a manner similar to paclitaxel (Taxol). Epothilones are produced from the myxobacterium Sorangium cellulosum, which has a 16-h doubling time and produces only milligram-per-liter amounts of epothilone A and epothilone B. Furthermore, genetic manipulation of S. cellulosum is difficult. To produce epothilones in a more genetically amenable and rapidly growing host, we chose the closely related and best-characterized myxobacteria Myxococcus xanthus. We inserted 65.4 kb of S. cellulosum DNA that encompassed the entire epothilone gene cluster into the chromosome of M. xanthus by a series of homologous recombination events. The resulting strain produced epothilones A and B. Construction of a strain that contained a mutation in epoK, the P450 epoxidase, resulted in production of epothilones C and D.

mlpB, a gene encoding a new lipoprotein in Myxococcus xanthus

AIMS

To search for and study the genes involved in the regulation of phosphate in the soil developmental bacterium Myxococcus xanthus.

METHODS AND RESULTS

The mlpB gene encoding a 149 residue polypeptide was identified while screening for genes with products related to phosphate metabolism. The amino terminal 19 residues of MlpB encode a typical prokaryotic signal sequence with a putative lipoprotein cleavage site.

CONCLUSIONS

In this study, a new myxobacterial putative lipoprotein is reported. The data suggest that MlpB may be involved in the secretion of phosphate-related proteins.

SIGNIFICANCE AND IMPACT OF THE STUDY

Soil bacteria have complex regulatory systems for using inorganic phosphate. This nutrient is limiting in the environment, and has a critical importance for growth and in the initiation of differentiation for developmental bacteria. A number of proteins are involved in all these processes, including membrane lipoproteins, which are being increasingly studied in M. xanthus.

Role for vitamin B(12) in light induction of gene expression in the bacterium Myxococcus xanthus

A light-inducible promoter (P(B)) drives the carB operon (carotenoid genes) of the bacterium Myxococcus xanthus. A gene encoding a regulator of carotenoid biosynthesis was identified by studying mutant strains carrying a transcriptional fusion to P(B) and deletions in three candidate genes. Our results prove that the identified gene, named carA, codes for a repressor of the P(B) promoter in the dark. They also show that the carA gene product does not participate in the light activation of two other promoters connected with carotenoid synthesis or its regulation in M. xanthus. CarA is a novel protein consisting of a DNA-binding domain of the family of MerR helix-turn-helix transcriptional regulators, directly joined to a cobalamin-binding domain. In support of this, we report here that the presence of vitamin B(12) or some other cobalamin derivatives is absolutely required for activation of the P(B) promoter by light.

Isolation of an Escherichia coil strain mutant unable to form biofilm on polystyrene and to adhere to human pneumocyte cells: involvement of tryptophanase

Escherichia coli adherence to biotic and abiotic surfaces constitutes the first step of infection by promoting colonization and biofilm formation. The aim of this study was to gain a better understanding of the relationship between E. coli adherence to different biotic surfaces and biofilm formation on abiotic surfaces. We isolated mutants defective in A549 pneumocyte cells adherence, fibronectin adherence, and biofilm formation by random transposition mutagenesis and sequential passages over A549 cell monolayers. Among the 97 mutants tested, 80 were decreased in biofilm formation, 8 were decreased in A549 cells adherence, 7 were decreased in their adherence to fibronectin, and 17 had no perturbations in either of the three phenotypes. We observed a correlation between adherence to fibronectin or A549 cells and biofilm formation, indicating that biotic adhesive factors are involved in biofilm formation by E. coli. Molecular analysis of the mutants revealed that a transposon insertion in the tnaA gene encoding for tryptophanase was associated with a decrease in both A549 cells adherence and biofilm formation by E. coli. The complementation of the tnaA mutant with plasmid-located wild-type tnaA restored the tryptophanase activity, epithelial cells adherence, and biofilm formation on polystyrene. The possible mechanism of tryptophanase involvement in E. coli adherence and biofilm formation is discussed.

ihfA gene of the bacterium Myxococcus xanthus and its role in activation of carotenoid genes by blue light

Myxococcus xanthus responds to blue light by producing carotenoids. Several regulatory genes are known that participate in the light action mechanism, which leads to the transcriptional activation of the carotenoid genes. We had already reported the isolation of a carotenoid-less, Tn5-induced strain (MR508), whose mutant site was unlinked to the indicated regulatory genes. Here, we show that OmegaMR508::Tn5 affects all known light-inducible promoters in different ways. It blocks the activation of two of them by light but makes the activity of a third one light independent. The OmegaMR508 locus has been cloned and sequenced. The mutation had occurred at the promoter of a gene we propose is the M. xanthus ortholog of ihfA. This encodes the alpha subunit of the histone-like integration host factor protein. An in-frame deletion within ihfA causes the same effects as the OmegaMR508::Tn5 insertion. Like other IhfA proteins, the deduced amino acid sequence of M. xanthus IhfA shows much similarity to HU, another histone-like protein. Sequence comparison data, however, and the finding that the M. xanthus gene is preceded by gene pheT, as happens in other gram-negative bacteria, strongly argue for the proposed orthology relationship. The M. xanthus ihfA gene shows some unusual features, both from structural and physiological points of view. In particular, the protein is predicted to have a unique, long acidic extension at the carboxyl terminus, and it appears to be necessary for normal cell growth and even vital for a certain wild-type strain of M. xanthus.

Molecular cloning and characterization of two genes for the biotin carboxylase and carboxyltransferase subunits of acetyl coenzyme A carboxylase in Myxococcus xanthus

We have cloned a DNA fragment from a genomic library of Myxococcus xanthus using an oligonucleotide probe representing conserved regions of biotin carboxylase subunits of acetyl coenzyme A (acetyl-CoA) carboxylases. The fragment contained two open reading frames (ORF1 and ORF2), designated the accB and accA genes, capable of encoding a 538-amino-acid protein of 58.1 kDa and a 573-amino-acid protein of 61.5 kDa, respectively. The protein (AccA) encoded by the accA gene was strikingly similar to biotin carboxylase subunits of acetyl-CoA and propionyl-CoA carboxylases and of pyruvate carboxylase. The putative motifs for ATP binding, CO(2) fixation, and biotin binding were found in AccA. The accB gene was located upstream of the accA gene, and they formed a two-gene operon. The protein (AccB) encoded by the accB gene showed high degrees of sequence similarity with carboxyltransferase subunits of acetyl-CoA and propionyl-CoA carboxylases and of methylmalonyl-CoA decarboxylase. Carboxybiotin-binding and acyl-CoA-binding domains, which are conserved in several carboxyltransferase subunits of acyl-CoA carboxylases, were found in AccB. An accA disruption mutant showed a reduced growth rate and reduced acetyl-CoA carboxylase activity compared with the wild-type strain. Western blot analysis indicated that the product of the accA gene was a biotinylated protein that was expressed during the exponential growth phase. Based on these results, we propose that this M. xanthus acetyl-CoA carboxylase consists of two subunits, which are encoded by the accB and accA genes, and occupies a position between prokaryotic and eukaryotic acetyl-CoA carboxylases in terms of evolution.

Characterization of an OprF-deficient mutant suggests that OprF is an essential protein for Pseudomonas fluorescens strain MF0

A stable OprF-deficient mutant for Pseudomonas fluorescens strain MF0 was constructed using reverse genetics. This mutant, called MF372, showed a rounded morphology and grew more slowly in minimal medium, but not in rich medium. Contrary to other Pseudomonas strains, the loss of OprF for strain MF0 was accompanied by an altered outer membrane composition. At least three outer membrane proteins were overexpressed, apparently as a consequence of adaptive mutations. The N-terminal sequence of two of them revealed strong similarities with porins of the OprD family from P. aeruginosa. The data presented here shows that OprF may be an essential protein for this P. fluorescens strain.

Spatial control of cell differentiation in Myxococcus xanthus

Myxococcus xanthus develops species-specific multicellular fruiting bodies. Starting from a uniform mat of cells, some cells enter into nascent fruiting body aggregates, whereas other cells remain outside. The cells within the fruiting body differentiate from rods into spherical, heat-resistant spores, whereas the cells outside the aggregates, called peripheral cells, remain rod-shaped. Early developmentally regulated genes are expressed in peripheral cells as well as by cells in the fruiting bodies. By contrast, late developmental genes are only expressed by cells within the nascent fruiting bodies. The data show that peripheral cells begin to develop, but are unable to express genes that are switched on later than about 6 h after the start of development. All of the genes whose expression is limited to the fruiting body are dependent on C-signaling either directly or indirectly, whereas the genes that are equally expressed in peripheral rods and in fruiting body cells are not. One of the C-signal-dependent and spatially patterned operons is called dev, and the dev operon has been implicated in the process of sporulation. It is proposed that expression of certain genes, including those of the dev operon, is limited to the nascent fruiting body because fruiting body cells engage in a high level of C-signaling. Peripheral cells do less C-signaling than fruiting body cells, because they have a different spatial arrangement and are at lower density. As a consequence, peripheral cells fail to express the late genes necessary for spore differentiation.

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.

Genetic studies of a thermoregulated gene in the psychrotrophic bacterium Pseudomonas fluorescens

In the psychrotrophic bacterium Pseudomonas fluorescens, some genes are thermoregulated: they are maximally expressed at a particular temperature within the broad range of temperatures that allow growth of this bacterium. To study this regulation, random transcriptional insertion fusions were obtained by means of mini-Tn5lacZ1 or mini-Tn5luxAB transposition. One fusion was studied in which beta-galactosidase production was maximal at a low-growth temperature. The mutated gene (that we call xsf) was highly homologous to xseA from Escherichia coli (and from other bacteria) which encodes the large subunit of exonuclease VII. Genetic tools were constructed in order to analyse and manipulate this fusion: a plasmid derived from R68.45 was used for chromosome transfer and a replacement vector was constructed to allow in situ marker exchange of the mini-Tn5lacZ1 by an Hg(r) interposon. This vector was used to make double mutants and hence to study the effect of the insertion in xsf on the expression of other fusions. Six genes were thereby identified with a decreased expression in an xsf- background and with different characteristics of thermoregulation.

A nonessential signal peptidase II (Lsp) of Myxococcus xanthus might be involved in biosynthesis of the polyketide antibiotic TA

Myxococcus xanthus is a gram-negative soil bacterium that produces the polyketide antibiotic TA. In this study, we describe the analysis of an M. xanthus gene which encodes a homologue of the prolipoprotein signal peptidase II (SPase II; lsp). Overexpression of the M. xanthus SPase II in Escherichia coli confers high levels of globomycin resistance, confirming its function as an SPase II. The M. xanthus gene encoding the lsp homologue is nonessential for growth, as determined by specific gene disruption. It has been mapped to the antibiotic TA gene cluster, and the disrupted mutants do not produce the antibiotic, indicating a probable involvement in TA production. These results suggest the existence of more than one SPase II protein in M. xanthus, where one is a system-specific SPase II (for TA biosynthesis).

Myxococcus cells respond to elastic forces in their substrate

Elasticotaxis describes the ability of Myxococcus xanthus cells to sense and to respond to elastic forces within an agar gel on which they rest. Within 5 min of the application of stress, each cell begins to reorient its long axis perpendicular to the stress force. The cells then glide in that direction, and the swarm becomes asymmetric. A quantifiable assay for the strength of elasticotaxis is based on the change in swarm shape from circular to elliptic. By using a collection of isogenic motility mutants, it has been found that the ability to respond to stress in agar depends totally on adventurous (A) motility, but not at all on social (S) motility or on the frz genes. In fact, S- mutants (which are moving only by means of A motility) respond to the applied stress more strongly than does the wild type, despite the fact that their spreading rates are slower than that of the wt strain. Based on the swarming and elasticotactic phenotypes of isogenic frizzy strains that were also defective either in A or S motility, frz behaves as if part of the S motility system.

Genetic and molecular analysis of cglB, a gene essential for single-cell gliding in Myxococcus xanthus

Gliding movements of individual isolated Myxococcus xanthus cells depend on the genes of the A-motility system (agl and cgl genes). Mutants carrying defects in those genes are unable to translocate as isolated cells on solid surfaces. The motility defect of cgl mutants can be transiently restored to wild type by extracellular complementation upon mixing mutant cells with wild-type or other motility mutant cells. To develop a molecular understanding of the function of a Cgl protein in gliding motility, we cloned the cglB wild-type allele by genetic complementation of the mutant phenotype. The nucleotide sequence of a 2.85-kb fragment was determined and shown to encode two complete open reading frames. The CglB protein was determined to be a 416-amino-acid putative lipoprotein with an unusually high cysteine content. The CglB antigen localized to the membrane fraction. The swarming and gliding defects of a constructed DeltacglB mutant were fully restored upon complementation with the cglB wild-type allele. Experiments with a cglB allele encoding a CglB protein with a polyhistidine tag at the C terminus showed that this allele also promoted wild-type levels of swarming and single-cell gliding, but was unable to stimulate DeltacglB cells to move. Possible functions of CglB as a mechanical component or as a signal protein in single cell gliding are discussed.

Cloning and characterization of a Myxococcus xanthus cytochrome P-450 hydroxylase required for biosynthesis of the polyketide antibiotic TA

The antibiotic TA, a complex macrocyclic polyketide of Myxococcus xanthus, is produced, like many other polyketides, through successive condensations of acetate by a type I polyketide synthase (PKS) mechanism. The chemical structure of this antibiotic and the mechanism by which it is synthesized indicate the need for several post-modification steps, such as a specific hydroxylation at C-20. Previous studies have shown that several genes, essential for TA biosynthesis, are clustered in a region of at least 36kb, which was subsequently cloned and analyzed. In this study, we report the analysis of a DNA fragment, containing a specific cytochrome P-450 hydroxylase, presumably responsible for the sole non-PKS hydroxylation at position C-20. Functional analysis of the cytochrome P-450 hydroxylase gene through specific gene disruption confirms that it is essential for the production of an active TA molecule.

Regulated exopolysaccharide production in Myxococcus xanthus

Myxococcus xanthus fibrils are cell surface-associated structures composed of roughly equal amounts of polysaccharide and protein. The level of M. xanthus polysaccharide production under different conditions in the wild type and in several mutants known to have alterations in fibril production was investigated. Wild-type exopolysaccharide increased significantly as cells entered the stationary phase of growth or upon addition of Ca2+ to growing cells, and the polysaccharide-induced cells exhibited an enhanced capacity for cell-cell agglutination. The activity of the key gluconeogenic pathway enzyme phosphoenolpyruvate carboxykinase (Pck) also increased under these conditions. Most fibril-deficient mutants failed to produce polysaccharide in a stationary-phase- or Ca2+-dependent fashion. However, regulation of Pck activity was generally unimpaired in these mutant strains. In an stk mutant, which overproduces fibrils, polysaccharide production and Pck activity were constitutively high under the conditions tested. Polysaccharide production increased in most fibril-deficient strains when an stk mutant allele was present, indicating that these fibril-deficient mutants retained the basic cellular components required for fibril polysaccharide production. In contrast to other divalent cations tested, Sr2+ effectively replaced Ca2+ in stimulating polysaccharide production, and either Ca2+ or Sr2+ was required for fruiting-body formation by wild-type cells. By using transmission electron microscopy of freeze-substituted log-phase wild-type cells, fibril material was observed as a cell surface-associated layer of uniform thickness composed of filaments with an ordered structure.

The first gene in the biosynthesis of the polyketide antibiotic TA of Myxococcus xanthus codes for a unique PKS module coupled to a peptide synthetase

The polyketide antibiotic TA is synthesized by the Gram negative bacterium Myxococcus xanthus in a multi-step process in which a unique glycine-derived molecule is used as a starter unit and elongated through the condensation of 11 acetate molecules by polyketide synthases (PKSs). Analysis of a 7.2 kb DNA fragment, encoding the protein that carries out the first condensation step, revealed that the fragment constitutes a single open reading frame, referred to as Ta1, which lacks the 5' and 3' ends and displays two regions of similarity to other proteins. The first 1020 amino acid residues at the N terminus of the polypeptide are similar to sequences of the large family of enzymes encoding peptide synthetases. They are followed by a second region displaying a high degree of similarity to type I PKS genes. The genetic analysis of this open reading frame is compatible with the proposed chemical structure of TA. The data indicate that the genes encoding TA have a modular gene organization, typical of a type I PKS system. The unusual feature of Ta1 is that the first PKS module of TA resides on the same polypeptide as the peptide synthetase functional unit.

The aadA gene of plasmid R100 confers resistance to spectinomycin and streptomycin in Myxococcus xanthus

Plasmids with the aadA gene from plasmid R100, which confers resistance to the aminoglycosides spectinomycin and streptomycin in Escherchia coli, can be introduced into wild-type Myxococcus xanthus, strain DK1622, by electroporation. Recombinant M. xanthus strains with integrated plasmids carrying the aadA gene acquire resistance to high levels of these antibiotics. Selection for aadA in M. xanthus can be carried out independently of, or simultaneously with, selection for resistance to kanamycin. The kinds and frequencies of recombination events observed between integrative plasmids with aadA and the M. xanthus chromosome are similar to those observed after the transformation of yeast. Cleavage of integrative plasmid DNA at a site adjacent to a region of homology between the plasmid and the M. xanthus genome favors the targeted disruption of M. xanthus genes by allele replacement.

Myxococcus xanthus sasN encodes a regulator that prevents developmental gene expression during growth

Myxococcus xanthus multicellular fruiting body development is initiated by nutrient limitation at high cell density. Five clustered point mutations (sasB5, -14, -15, -16, and -17) can bypass the starvation and high-cell-density requirements for expression of the 4521 developmental reporter gene. These mutants express 4521 at high levels during growth and development in an asgB background, which is defective in generation of the cell density signal, A signal. A 1.3-kb region of the sasB locus cloned from the wild-type chromosome restored the SasB+ phenotype to the five mutants. DNA sequence analysis of the 1.3-kb region predicted an open reading frame, designated SasN. The N terminus of SasN appears to contain a strongly hydrophobic region and a leucine zipper motif. SasN showed no significant sequence similarities to known proteins. A strain containing a newly constructed sasN-null mutation and Omega4521 Tn5lac in an otherwise wild-type background expressed 4521 at a high level during growth and development. A similar sasN-null mutant formed abnormal fruiting bodies and sporulated at about 10% the level of wild type. These data indicate that the wild-type sasN gene product is necessary for normal M. xanthus fruiting body development and functions as a critical regulator that prevents 4521 expression during growth.

Targeted mutagenesis of sigma54 activator proteins in Myxococcus xanthus

Myxococcus xanthus DNA segments related to the highly conserved central sequence of sigma54 activator proteins have been investigated. A genetic technique designed to inactivate a gene that encodes such an activator by inserting a plasmid-borne internal fragment of the putative gene has been tested. When the internal fragment inserted by homologous recombination into the corresponding chromosomal locus, the expected duplication of the gene was observed by Southern hybridization. The single restriction fragment characteristic of each segment was replaced in the insertion strains by two hybridizing fragments, and one of these fragments hybridized with the kanamycin resistance gene of the plasmid vector. The combined molecular weights of the two fragments from the insertion strains were equal to the molecular weight of the original fragment plus the expected molecular weight contributed by the vector. In the duplication, one copy is expected to have an N-terminal deletion and the other copy is expected to have a C-terminal deletion. In most cases, the net result should be loss of activator function. If an activator is essential for vegetative growth, then it should not be possible to obtain the insertion strain by plasmid integration. Indeed, integrants for three of the segments were not obtained in repeated trials; however, a plausible explanation for these results other than lethality can be offered. Of the seven insertions validated by Southern hybridization, four strains exhibited defects in the development of fruiting bodies. One of these failed to develop in submerged culture, though it developed normally on agar. The other three showed arrested development of fruiting bodies, each at a morphologically different stage of aggregation. One of the mutants may be defective in the reception pathway of A-signal.

Molecular analysis of the DNA gyrB gene from Myxococcus xanthus

DNA gyrase, an essential type II topoisomerase, mediates negative supercoiling of the bacterial chromosome, thereby affecting the processes of DNA replication, transcription, recombination and repair. The gyrB gene from the Gram-negative soil bacterium Myxococcus xanthus was sequenced. The sequence predicts a protein of 815 amino acid residues displaying significant homology to all known GyrB proteins. A 6-His-GyrB fusion protein was overexpressed in Escherichia coli and purified to near homogeneity using affinity chromatography on Ni-nitrilotriacetic acid-agarose and novobiocin-Sepharose columns. The fusion protein bound novobiocin and cross-reacted with anti-E. coli GyrB antibodies, indicating structural and functional similarities to the E. coli DNA GyrB. The gene was mapped to the region of the origin of replication (oriC) of M. xanthus.

A new sigma factor, SigD, essential for stationary phase is also required for multicellular differentiation in Myxococcus xanthus

BACKGROUND

Myxococcus xanthus is a gram-negative bacterium that undergoes spectacular development to form multicellular fruiting bodies under nutrient deprivation. Inside a fruiting body, vegetative cells differentiate into spores. A number of sigma factors have been shown to play roles in the regulation of gene expression in the M. xanthus life cycle. Additional sigma factors were searched to further explore the M. xanthus life cycle.

RESULTS

A new sigma factor was identified, SigD, which consists of 297 amino acid residues. Two transcription initiation sites for the sigD gene were detected by primer extension analysis using total RNA from the vegetative and developmental cells, one of which was specific for development. The characterization of sigD-lacZ fusion strains demonstrated that sigD expression increased during entry into stationary phase of vegetative growth and during early development. A deletion mutant of sigD exhibited growth defects during the late-log phase and stationary phase, with dramatically reduced cell viability. The patterns of protein synthesis at late log phase of vegetative growth and at early development on CF agar plates were significantly different between the deletion mutant and the wild-type strain. The deletion mutant was more sensitive to various stresses when compared with the wild-type strain and did not accumulate trehalose in response to osmotic stress. The deletion mutant also showed a significant delay in fruiting body formation and sporulation and yielded fewer spores than the wild-type strain.

CONCLUSIONS

SigD shows characteristic features of the stationary phase sigma factors and also plays important roles in multicellular differentiation of M. xanthus.

Identification of the omega4400 regulatory region, a developmental promoter of Myxococcus xanthus

Omega4400 is the site of a Tn5 lac insertion in the Myxococcus xanthus genome that fuses lacZ expression to a developmentally regulated promoter. Cell-cell interactions that occur during development, including C signaling, are required for normal expression of Tn5 lac omega4400. The DNA upstream of the omega4400 insertion has been cloned, the promoter has been localized, and a partial open reading frame has been identified. From the deduced amino acid sequence of the partial open reading frame, the gene disrupted by Tn5 lac omega4400 may encode a protein with an ATP- or GTP-binding site. Expression of the gene begins 6 to 12 h after starvation initiates development, as measured by beta-galactosidase production in cells containing Tn5 lac omega4400. The putative transcriptional start site was mapped, and deletion analysis has shown that DNA downstream of -101 bp is sufficient for C-signal-dependent, developmental activation of this promoter. A deletion to -76 bp eliminated promoter activity, suggesting the involvement of an upstream activator protein. The promoter may be transcribed by RNA polymerase containing a novel sigma factor, since a mutation in the M. xanthus sigB or sigC gene did not affect Tn5 lac omega4400 expression and the DNA sequence upstream of the transcriptional start site did not match the sequence of any M. xanthus promoter transcribed by a known form of RNA polymerase. However, the omega4400 promoter does contain the sequence 5'-CATCCCT-3' centered at -49 and the C-signal-dependent omega4403 promoter also contains this sequence at the same position. Moreover, the two promoters match at five of six positions in the -10 regions, suggesting that these promoters may share one or more transcription factors. These results begin to define the cis-acting regulatory elements important for cell-cell interaction-dependent gene expression during the development of a multicellular prokaryote.

A DnaK homolog in Myxococcus xanthus is involved in social motility and fruiting body formation

Myxococcus xanthus is a gram-negative soil bacterium which exhibits a complex life cycle and social behavior. In this study, two developmental mutants of M. xanthus were isolated through Tn5 transposon mutagenesis. The mutants were found to be defective in cellular aggregation as well as in sporulation. Further phenotypic characterization indicated that the mutants were defective in social motility but normal in directed cell movements. Both mutations were cloned by a transposon-tagging method. Sequence analysis indicated that both insertions occurred in the same gene, which encodes a homolog of DnaK. Unlike the dnaK genes in other bacteria, this M. xanthus homolog appears not to be regulated by temperature or heat shock and is constitutively expressed during vegetative growth and under starvation. The defects of the mutants indicate that this DnaK homolog is important for the social motility and development of M. xanthus.

Propionyl coenzyme A carboxylase is required for development of Myxococcus xanthus

A dcm-1 mutant, obtained by transposon mutagenesis of Myxococcus xanthus, could aggregate and form mounds but was unable to sporulate under nutrient starvation. A sequence analysis of the site of insertion of the transposon showed that the insertion lies within the 3' end of a 1,572-bp open reading frame (ORF) designated the M. xanthus pccB ORF. The wild-type form of the M. xanthus pccB gene, obtained from a lambdaEMBL library of M. xanthus, shows extensive similarity to a beta subunit of propionyl coenzyme A (CoA) carboxylase, an alpha subunit of methylmalonyl-CoA decarboxylase, and a 12S subunit of transcarboxylase. In enzyme assays, extracts of the dcm-1 mutant were deficient in propionyl-CoA carboxylase activity. This enzyme catalyzes the ATP-dependent carboxylation of propionyl-CoA to yield methylmalonyl-CoA. The methylmalonyl-CoA rescued the dcm-1 mutant fruiting body and spore development. During development, the dcm-1 mutant cells also had reduced levels of long-chain fatty acids (C16 to C18) compared to wild-type cells.

The tgl gene: social motility and stimulation in Myxococcus xanthus

Mutations in the tgl locus inactivate social gliding motility in Myxococcus xanthus and block production of pili. The tgl locus is distinctive among the genes for social motility because social gliding and pili can be restored transiently to tgl mutant cells by mixing them with tgl+ cells, a process known as stimulation. The tgl locus was cloned with a linked insertion of transposon Tn5 by using the kanamycin resistance encoded by that transposon. A 16-kb segment of chromosomal DNA complemented the social motility defect when introduced into tgl mutant cells to form a tandem duplication tgl+/tgl heterozygote. To delimit the autonomous tgl transcription unit, subfragments of this 16-kb piece were integrated at the ectopic Mx8 prophage attachment site. A 1.7-kb DNA fragment was identified which, when integrated at the Mx8 site, simultaneously rescued social motility and pilus production. The ability to stimulate tgl mutants was also rescued by the 1.7-kb fragment. Because rescue of stimulation from an mgl-deficient donor strain which cannot swarm was observed, this demonstrates that a stimulation donor requires a tgl+ allele but does not require the capacity to swarm actively. The nucleotide sequence of the 1.7-kb fragment revealed two protein coding regions, open reading frame A and open reading frame B (ORFB). ORFB is the tgl gene, because a 613-bp DNA fragment which includes 75% of ORFB rescues tgl-1, -2, and -3 mutants and because disruption of ORFB by deletion or insertion of transposon Tn5lac constitutes a tgl mutation.

Identification and characterization of Myxococcus xanthus mutants deficient in calcofluor white binding

Calcofluor white is a fluorescent dye that binds to glycans and can be used to detect extracellular polysaccharide in Myxococcus xanthus and many other bacteria. We observed that an esg mutant showed less binding to calcofluor white than wild-type cells. Unlike S-motility mutants that share this phenotypic characteristic, the esg mutant exhibited S motility. This led us to identify a collection of nine new transposon insertion mutants, designated Cds (for calcofluor white binding deficient and S motile), which exhibited a phenotype similar to that of the esg strain. The Cds phenotype was found in 0.6% of the random insertion mutants that were screened. The Cds mutants were also found to be defective in cell-cell agglutination and developmental aggregation. Extracellular matrix fibrils composed of roughly equal amounts of polysaccharide and protein have been shown to be involved in agglutination, and electron microscopic examination showed that esg and the other Cds mutants lack the wild-type level of fibrils. Analysis of total M. xanthus carbohydrate demonstrated that polysaccharide content increased by about 50% when wild-type cells entered stationary phase. This induction was reduced or eliminated in all of the Cds mutants. The degree of polysaccharide deficiency in the Cds mutants correlated with the degree of loss of agglutination and dye binding as well as with the severity of the developmental aggregation defect. Preliminary genetic characterization demonstrated that the transposon insertion mutations in three of the Cds mutants (SR53, SR171, and SR200) were loosely linked. The results of this study suggest that many genes are involved in the production of calcofluor white binding polysaccharide material found in the extracellular matrix and that the polysaccharide is fibrillar. These results are also consistent with the findings of earlier studies which indicated that fibrils function to join agglutinating cells and to form multicellular fruiting aggregates.

sigma54, a vital protein for Myxococcus xanthus

The rpoN gene encoding the transcription factor sigma54 in Myxococcus xanthus has been cloned using a heterologous rpoN probe. The sequence of the cross-hybridizing DNA confirmed the existence of an ORF 1518 bp long that encodes a well conserved member of the sigma54 family of sigma factors. Low- as well as high-stringency hybridizations detected only a single rpoN gene in the M. xanthus chromosome. In other bacteria, sigma54 is an alternative sigma, and null mutants are viable. However, all attempts to construct a strain containing a null mutation in the M. xanthus rpoN have been unsuccessful. Partial diploids of rpoN+/rpoN null are viable. Recombination experiments with such partial diploids showed the impossibility of constructing, either by segregation or by transduction, a viable null haploid under any of a wide range of growth conditions. The product of the rpoN gene, sigma54, therefore appears to be essential for growth in M. xanthus.

Characterization of the regulatory region of a cell interaction-dependent gene in Myxococcus xanthus

omega 4403 is the site of a Tn5 lac insertion in the Myxococcus xanthus genome that fuses lacZ expression to a developmentally regulated promoter. Cell-cell interactions that occur during development, including C-signaling, are required for expression of Tn5 lac omega 4403. We have cloned DNA upstream of the omega 4403 insertion site, localized the promoter, and identified a potential open reading frame. From the deduced amino acid sequence, the gene disrupted by Tn5 lac omega 4403 appears to encode a serine protease that is dispensable for development. The gene begins to be expressed between 6 and 12 h after starvation initiates development, as determined by measuring mRNA or beta-galactosidase accumulation in cells containing Tn5 lac omega 4403. The putative transcriptional start site was mapped, and sequences centered near -10 and -35 bp relative to this site show some similarity to the corresponding regions of promoters transcribed by Escherichia coli sigma70 RNA polymerase. However, deletions showed that an essential promoter element lies between -80 and -72 bp, suggesting the possible involvement of an upstream activator protein. DNA downstream of -80 is sufficient for C-signal-dependent activation of this promoter. The promoter is not fully expressed when fusions are integrated at the Mx8 phage attachment site in the chromosome. Titration of a limiting factor by two copies of the regulatory region (one at the attachment site and one at the native site) can, in part, explain the reduced expression. We speculate that the remaining difference may be due to an effect of chromosomal position. These results provide a basis for studies aimed at identifying regulators of C-signal-dependent gene expression.

Intercellular C-signaling in Myxococcus xanthus involves a branched signal transduction pathway

C-factor, the product of the csgA gene, is a cell-surface associated short-range intercellular signaling protein in Myxococcus xanthus. C-factor is required for at least four responses during starvation-induced fruiting body morphogenesis: rippling, aggregation, sporulation, and full expression of the csgA gene, all of which fail in a csgA mutant. To analyze the C-factor signaling pathway, eight Tn5 lac insertion mutants that began but failed to complete fruiting body aggregation were characterized. Seven of the insertions identified genes whose products function in the csgA signaling pathway. The seven mutants were differentially deficient in the C-factor responses, and could be divided into two classes on the basis of those differences. On one hand, the four mutants in class I were deficient in rippling and aggregation, but sporulated and produced C-factor at wild-type levels. The Tn5 lac insertions in the class I mutants mapped to the frz locus, which encodes a signal transduction system that controls the frequency of single cell reversals. On the other hand, mutants carrying any of the three closely linked class II Tn5 lac insertions had deficiencies in all four C-factor responses. Because the sporulation defect in the class 11 mutants is cell autonomous, the data suggest that the primary defect in these mutants is an inability to respond to the C-factor signal. All the data can be explained by a model in which the first part of the C-factor signaling pathway is common to all four C-factor-dependent responses. The genes identified by the class 11 insertions would function in the common part. Downstream of class II, the pathway branches. One branch includes the frz genes and leads to aggregation and rippling; the second branch leads to sporulation and controls the level of csgA gene expression. This model was confirmed in epistasis tests with characterized frz mutations, a csgA null mutation, and a class II mutation.

Using a phase-locked mutant of Myxococcus xanthus to study the role of phase variation in development

The bacterium Myxococcus xanthus undergoes a primitive developmental cycle in response to nutrient deprivation. The cells aggregate to form fruiting bodies in which a portion of the cells differentiate into environmentally resistant myxospores. During the growth portion of the M. xanthus life cycle, the organism also undergoes a phase variation, in which cells alternate between yellow and tan colony-forming variants. Phase variation occurs in our laboratory strain (M102, a derivative of DK1622) at a frequency high enough that a single colony of either the yellow or the tan phase already contains cells of the alternate phase. In this study we demonstrate that tan cells within a predominantly yellow population of phase variation-proficient cells are preferentially recovered as heat- and sonication-resistant spores. To further investigate the possibility of a differential role of tan and yellow cells during development, a tan-phase-locked mutant was used to compare the developmental phenotypes of a pure tan population with a predominantly yellow, phase variation-proficient population. Pure tan-phase populations did not produce fruiting bodies or mature spores under conditions in which predominantly yellow wild-type populations did so efficiently. Pure populations of tan-phase cells responded to developmental induction by changing from vegetative rod-shaped cells to round forms but were unable to complete the maturation to heat- and sonication-resistant, refractile spores. The developmental defect of a tan-phase-locked mutant was rescued by the addition of phase variation-proficient cells from a predominantly yellow culture. In such mixtures the tan-phase-locked mutant not only completed the process of forming spores but also was again preferentially represented among the viable spores. These findings suggest the intriguing possibility that the tan-phase cells within the vegetative population entering development are the progenitors of spores and implicate a requirement for yellow-phase cells in spore maturation.

Myxococcus xanthus, a gram-negative bacterium, contains a transmembrane protein serine/threonine kinase that blocks the secretion of beta-lactamase by phosphorylation

A gene, pkn2, encoding a Myxococcus xanthus protein with significant similarities to eukaryotic protein serine/threonine kinases, was cloned using the polymerase chain reaction. The open reading frame for the protein, beginning with a GUG initiation codon, consists of 830 amino acids. The amino-terminal 279 residues show 37% identity to catalytic domain of Pkn1, another protein serine/threonine kinase expressed during the development at the onset of sporulation. The catalytic domain of Pkn2 contains 27% and 25% identity to rat Ca2+/calmodulin-dependent protein kinase and Bos taurus rhodopsin kinase, respectively. In the middle of the carboxy-terminal regulatory domain, there is a typical transmembrane domain consisting of 18 hydrophobic residues. The gene product, Pkn2, produced in Escherichia coli under a T7 promoter was phosphorylated at both serine and threonine residues. TEM-beta-lactamase produced in E. coli was found to serve as an effective substrate for Pkn2, phosphorylated only at threonine residues, shifting its apparent molecular mass from 29 to 44 kD. The phosphorylated beta-lactamase was unable to be secreted into the periplasmic space and localized in the cytoplasmic and membrane fractions. Analysis of phoA fusions with pkn2 demonstrated that Pkn2 is a transmembrane protein with the kinase domain in the cytoplasm and the 207-residue carboxy-terminal domain outside the cytoplasmic membrane. Disruption of pkn2 showed no effect on vegetative growth but reduced the yield of myxospores by 30%-50%. On the basis of the present results, we propose that Pkn2 is a transmembrane protein serine/threonine kinase that regulates the activity of endogenous beta-lactamase or related enzymes in response to an external signal yet to be identified.

The Myxococcus xanthus asgA gene encodes a novel signal transduction protein required for multicellular development

The Myxococcus xanthus asgA gene is one of three known genes necessary for the production of extracellular A-signal, a cell density signal required early in fruiting body development. We determined the DNA sequence of asgA. The deduced 385-amino-acid sequence of AsgA was found to contain two domains: one homologous to the receiver domain of response regulators and the other homologous to the transmitter domain of histidine protein kinases. A kanamycin resistance (Kmr) gene was inserted at various positions within or near the asgA gene to determine the null phenotype. Those strains with the Kmr gene inserted upstream or downstream of asgA are able to form fruiting bodies, while strains containing the Kmr gene inserted within asgA fail to develop. The nature and location of the asgA476 mutation were determined. This mutation causes a leucine-to-proline substitution within a conserved stretch of hydrophobic residues in the N-terminal receiver domain. Cells containing the insertion within asgA and cells containing the asgA476 substitution have similar phenotypes with respect to development, colony color, and expression of an asg-dependent gene. An analysis of expression of a translational asgA-lacZ fusion confirms that asgA is expressed during growth and early development. Finally, we propose that AsgA functions within a signal transduction pathway that is required to sense starvation and to respond with the production of extracellular A-signal.

A genetic link between light response and multicellular development in the bacterium Myxococcus xanthus

The Gram-negative bacterium Myxococcus xanthus responds to blue light by producing carotenoid pigments (Car+ phenotype). Genes for carotenoid synthesis lie at two unlinked chromosomal sites, the carC and the carBA operon, but are integrated in a single "light regulon" by the action of common trans-acting regulatory elements. Three known regulatory genes are grouped together at the (light-inducible) carQRS operon. By screening the Car phenotype of a large collection of transposon-induced mutants, we have identified a new car locus that has been named carD (carD1 for the mutant allele). The carD gene product plays a critical role in the light regulon, as it is required for activation of the carQRS and carC promoters by blue light. The carD1 mutant is impaired in the (starvation-induced) developmental process that allows M. xanthus cells both to form multicellular fruiting bodies and to sporulate. Our results indicate that the carD gene product is also required for the expression of a particular set of development-specific genes that are normally activated through the action of intercellular signals.

Evidence that asgB encodes a DNA-binding protein essential for growth and development of Myxococcus xanthus

The asg mutants of Myxococcus xanthus are defective in production of extracellular A-signal, which serves as a cell density signal for fruiting-body development. The DNA sequence of asgB, one of the three asg genes, was determined. The deduced amino acid sequence of AsgB contains a DNA-binding helix-turn-helix motif near the C terminus. This putative helix-turn-helix is highly similar to the helix-turn-helix in region 4.2 of major sigma factors, which is the region that recognizes and interacts with -35 sequences of promoters. We propose that AsgB is a transcription factor that binds to DNA sequences similar to the -35 hexamer, TTGACA. Analyses of asgB RNA levels and expression of an asgB-lacZ translational fusion indicate that expression of asgB remains fairly constant during the transition from growth into early development. The mutation within the asgB480 allele was identified as an A-to-G transition that results in a threonine-to-alanine substitution in the predicted protein product. Attempts to replace the wild-type copy of asgB with a null allele failed, indicating that asgB may be essential for growth.

Identification of the Myxococcus xanthus 59-kDa membrane-associated GTP-binding protein as a proton-translocating ATPase

Five GTP-binding proteins have been detected in Myxococcus xanthus by photoaffinity cross-linking with azido-GTP [Muñoz-Dorado et al., J. Biol. Chem. 265 (1990a) 2702-2706]. One of them, the 59-kDa membrane-associated GTP-binding protein, has been purified. The N-terminal sequence of a 10-kDa fragment from the protease V8 digestion of the purified protein has been determined and degenerate oligodeoxyribonucleotides based on that sequence have been used to isolate and clone the gene that encodes the GTP-binding protein. The gene was sequenced and further analysis of the sequence revealed that the protein encoded by this gene shows very high homology with the alpha subunit of proton-translocating ATPases.

Identification of esg, a genetic locus involved in cell-cell signaling during Myxococcus xanthus development

JD258, a Tn5 insertion mutant of Myxococcus xanthus, was shown to have major defects in three development-associated properties: expression of the developmentally regulated tps gene, spore formation, and production of multicellular fruiting bodies. The defects in tps gene expression and sporulation could be substantially corrected, at the phenotypic level, by mixing JD258 with wild-type cells (extracellular complementation). By this criterion, JD258 appeared to be a new member of a group of conditional developmental mutants that were previously characterized and placed in four extracellular complementation groups (A to D) based on the ability of mutants in one group to stimulate development in mutants belonging to a different group (D. C. Hagen, A. P. Bretscher, and D. Kaiser, Dev. Biol. 64:284-296, 1978). Mutants from groups A, B, C, and D all displayed extracellular complementation activity when mixed with JD258. These results, and other aspects of the phenotype of JD258, indicate that this mutant defines a fifth extracellular complementation group, group E. The M. xanthus esg locus identified by the Tn5 insertion in JD258 was cloned in Escherichia coli and used for further genetic analysis of the locus. These studies indicated that the esg locus resides within a 2.5-kb region of the M. xanthus chromosome and that the locus contains at least two genetic complementation groups. Our results are consistent with a model in which the esg locus controls the production of a previously unrecognized extracellular signal that must be transmitted between cells for the completion of M. xanthus development.

devRS, an autoregulated and essential genetic locus for fruiting body development in Myxococcus xanthus

Two Tn5 lac insertions into the Myxococcus genome at sites omega 4414 and omega 4473, which are separated by 550 nucleotides, inactivate fruiting body development. Sporulation is decreased 100- to 10,000-fold. At least two genes, devR and devS, are transcribed in this region, probably as an operon. Expression of devR begins by 6 h after starvation has initiated development. On the basis of their nucleotide sequences, devR and devS are expected to encode proteins of 302 and 214 amino acids, respectively. Dev+ function can be restored by a segment of 7.8 kb cloned from the devRS region of wild-type cells. Two experiments show that devR expression is under strong negative autoregulation. beta-Galactosidase is expressed at a higher level from a transcriptional devR::lacZ fusion when the fused operon is in a dev strain than when it is in the dev/dev+ genetic background of a partial diploid. There is more mRNA accumulation from the devRS region in the dev strain than in a rescued dev/dev+ tandem duplication strain. Sporulation rescue is correlated with some degree of negative autoregulation, even though sporulation is not inversely proportional to beta-galactosidase expression from omega 4414. A second level of regulation is suggested by complementation of dev by dev+ in duplication strains. The expression of devRS, measured by sporulation levels, differs 1,000-fold when devRS+ is moved from a distance of 20 kb to 3 Mb from the mutant devRS locus. Expression of devR is also dependent on the cell density at which development is initiated, a third level of regulation. Multiple levels of regulation suggest that devRS is a switch required to activate completion of aggregation and sporulation.