The Myxococcus xanthus pilQ (sglA) gene encodes a secretin homolog required for type IV pilus biogenesis, social motility, and development

Identification and characterization of spdR mutations that bypass the BsgA protease-dependent regulation of developmental gene expression in Myxococcus xanthus

The BsgA protease of Myxococcus xanthus is an intracellular protease closely related to the Lon protease of Escherichia coli. BsgA is required for normal levels of developmentally induced gene expression. In this report, we describe the identification of mutations that suppress the developmental defect of bsgA mutants. These mutations localized to the spdR gene (suppressor protease deficiency regulator) that appears to play a role in the regulation of early developmental gene expression. Mutations in spdR fully restored the ability of a bsgA mutant to form fruiting bodies and spores and, with one exception, restored the expression of several development-specific lacZ fusions. spdR mutants exhibited characteristic phenotypic properties including increased expression of the development-specific tps gene during vegetative growth, formation of fruiting bodies and spores on semi-rich nutrient medium and completion of starvation-induced development in a shorter time period than wild-type strains. The spdR locus was cloned and sequenced and found to encode a member of the NtrC family of two-component transcriptional regulators. One interpretation of these data is that SpdR acts, directly or otherwise, to regulate developmental gene expression negatively and that the BsgA protease is required to relieve this inhibitory effect at the onset of development. However, Western immunoblot analysis indicated that SpdR is present at a relatively constant level during growth and early development in both wild-type and BsgA protease-deficient cells. This finding suggests that BsgA does not function to degrade SpdR at the onset of development.

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.

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

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

Identification and characterization of genes required for early Myxococcus xanthus developmental gene expression

Starvation and cell density regulate the developmental expression of Myxococcus xanthus gene 4521. Three classes of mutants allow expression of this developmental gene during growth on nutrient agar, such that colonies of strains containing a Tn5 lac Omega4521 fusion are Lac(+). One class of these mutants inactivates SasN, a negative regulator of 4521 expression; another class activates SasS, a sensor kinase-positive regulator of 4521 expression; and a third class blocks lipopolysaccharide (LPS) O-antigen biosynthesis. To identify additional positive regulators of 4521 expression, 11 Lac(-) TnV. AS transposon insertion mutants were isolated from a screen of 18,000 Lac(+) LPS O-antigen mutants containing Tn5 lac Omega4521 (Tc(r)). Ten mutations identified genes that could encode positive regulators of 4521 developmental expression based on their ability to abolish 4521 expression during development in the absence of LPS O antigen and in an otherwise wild-type background. Eight of these mutations mapped to the sasB locus, which encodes the known 4521 regulators SasS and SasN. One mapped to sasS, whereas seven identified new genes. Three mutations mapped to a gene encoding an NtrC-like response regulator homologue, designated sasR, and four others mapped to a gene designated sasP. One mutation, designated ssp10, specifically suppressed the LPS O-antigen defect; the ssp10 mutation had no effect on 4521 expression in an otherwise wild-type background but reduced 4521 developmental expression in the absence of LPS O antigen to a level close to that of the parent strain. All of the mutations except those in sasP conferred defects during growth and development. These data indicate that a number of elements are required for 4521 developmental expression and that most of these are necessary for normal growth and fruiting body development.

AglU, a protein required for gliding motility and spore maturation of Myxococcus xanthus, is related to WD-repeat proteins

The aglU gene of Myxococcus xanthus encodes a protein similar to Het-E1 (vegetative incompatibility) from Podospora anserina, acylaminoacyl-peptidase from Bacillus subtilis, and TolB from Escherichia coli. These proteins all have evenly spaced SPDG repeats that are characteristic of a larger motif called the WD-repeat. The WD-repeat is predicted to form a beta-propeller structure that mediates the assembly of heteromeric protein complexes. AglU has a consensus lipoprotein attachment motif that includes a type II signal sequence followed by a cysteine residue. This suggests that AglU is matured, then attached to the outer membrane via fatty acid acylation at this Cys. Cells carrying a mutation in aglU are blocked in adventurous gliding and can swarm only if cells are in contact with one another. When starved of nutrients, the aglU mutant aggregates and forms multicellular fruiting bodies like the wild-type strain, but is unable to produce heat-resistant spores. This suggests that adventurous gliding motility, per se, is not required for development, but that AglU is essential for a terminal step of spore differentiation.

Developmental cheating in the social bacterium Myxococcus xanthus

Cheating is a potential problem in any social system that depends on cooperation and in which actions that benefit a group are costly to individuals that perform them. Genetic mutants that fail to perform a group-beneficial function but that reap the benefits of belonging to the group should have a within-group selective advantage, provided that the mutants are not too common. Here we show that social cheating exists even among prokaryotes. The bacterium Myxococcus xanthus exhibits several social behaviours, including aggregation of cells into spore-producing fruiting bodies during starvation. We examined a number of M. xanthus genotypes that were defective for fruiting-body development, including several lines that evolved for 1,000 generations under asocial conditions and others carrying defined mutations in developmental pathways, to determine whether they behaved as cheaters when mixed with their developmentally proficient progenitor. Clones from several evolved lines and two defined mutants exhibited cheating during development, being overrepresented among resulting spores relative to their initial frequency in the mixture. The ease of finding anti-social behaviours suggests that cheaters may be common in natural populations of M. xanthus.

Disruption of aldA influences the developmental process in Myxococcus xanthus

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

Motility in Myxococcus xanthus and its role in developmental aggregation

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

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

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

A re-examination of twitching motility in Pseudomonas aeruginosa

Twitching motility is a form of solid surface translocation which occurs in a wide range of bacteria and which is dependent on the presence of functional type IV fimbriae or pili. A detailed examination of twitching motility in Pseudomonas aeruginosa under optimal conditions in vitro was carried out. Under these conditions (at the smooth surface formed between semi-solid growth media and plastic or glass surfaces) twitching motility is extremely rapid, leading to an overall radial rate of colony expansion of 0.6 mm h(-1) or greater. The zones of colony expansion due to twitching motility are very thin and are best visualized by staining. These zones exhibit concentric rings in which there is a high density of microcolonies, which may reflect periods of expansion and consolidation/cell division. Video microscopic analysis showed that twitching motility involves the initial formation of large projections or rafts of aggregated cells which move away from the colony edge. Behind the rafts, individual cells move rapidly up and down trails which thin and branch out, ultimately forming a fine lattice-like network of cells. The bacteria in the lattice network then appear to settle and divide to fill out the colonized space. Our observations redefine twitching motility as a rapid, highly organized mechanism of bacterial translocation by which P. aeruginosa can disperse itself over large areas to colonize new territories. It is also now clear, both morphologically and genetically, that twitching motility and social gliding motility, such as occurs in Myxococcus xanthus, are essentially the same process.

Gliding motility in bacteria: insights from studies of Myxococcus xanthus

Gliding motility is observed in a large variety of phylogenetically unrelated bacteria. Gliding provides a means for microbes to travel in environments with a low water content, such as might be found in biofilms, microbial mats, and soil. Gliding is defined as the movement of a cell on a surface in the direction of the long axis of the cell. Because this definition is operational and not mechanistic, the underlying molecular motor(s) may be quite different in diverse microbes. In fact, studies on the gliding bacterium Myxococcus xanthus suggest that two independent gliding machineries, encoded by two multigene systems, operate in this microorganism. One machinery, which allows individual cells to glide on a surface, independent of whether the cells are moving alone or in groups, requires the function of the genes of the A-motility system. More than 37 A-motility genes are known to be required for this form of movement. Depending on an additional phenotype, these genes are divided into two subclasses, the agl and cgl genes. Videomicroscopic studies on gliding movement, as well as ultrastructural observations of two myxobacteria, suggest that the A-system motor may consist of multiple single motor elements that are arrayed along the entire cell body. Each motor element is proposed to be localized to the periplasmic space and to be anchored to the peptidoglycan layer. The force to glide which may be generated here is coupled to adhesion sites that move freely in the outer membrane. These adhesion sites provide a specific contact with the substratum. Based on single-cell observations, similar models have been proposed to operate in the unrelated gliding bacteria Flavobacterium johnsoniae (formerly Cytophaga johnsonae), Cytophaga strain U67, and Flexibacter polymorphus (a filamentous glider). Although this model has not been verified experimentally, M. xanthus seems to be the ideal organism with which to test it, given the genetic tools available. The second gliding motor in M. xanthus controls cell movement in groups (S-motility system). It is dependent on functional type IV pili and is operative only when cells are in close proximity to each other. Type IV pili are known to be involved in another mode of bacterial surface translocation, called twitching motility. S-motility may well represent a variation of twitching motility in M. xanthus. However, twitching differs from gliding since it involves cell movements that are jerky and abrupt and that lack the organization and smoothness observed in gliding. Components of this motor are encoded by genes of the S-system, which appear to be homologs of genes involved in the biosynthesis, assembly, and function of type IV pili in Pseudomonas aeruginosa and Neisseria gonorrhoeae. How type IV pili generate force in S-motility is currently unknown, but it is to be expected that ongoing physiological, genetic, and biochemical studies in M. xanthus, in conjunction with studies on twitching in P. aeruginosa and N. gonorrhoeae, will provide important insights into this microbial motor. The two motility systems of M. xanthus are affected to different degrees by the MglA protein, which shows similarity to a small GTPase. Bacterial chemotaxis-like sensory transduction systems control gliding motility in M. xanthus. The frz genes appear to regulate gliding movement of individual cells and movement by the S-motility system, suggesting that the two motors found in this bacterium can be regulated to result in coordinated multicellular movements. In contrast, the dif genes affect only S-system-dependent swarming.

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.

Type IV pili and cell motility

Type IV pili (Tfp) mediate the movement of bacteria over surfaces without the use of flagella. These movements are known as social gliding in Myxococcus xanthus and twitching in organisms such as Pseudomonas aeruginosa and Neisseria gonorrhoeae. Tfp are localized polarly. Type IV pilins have a signature N-terminal domain, which forms a coiled-coil with other monomer units to polymerize a pilus fibre. At least 10 more proteins at the base of the fibre are conserved; they are related to the type II secretion system. Movements produced by Tfp range from short, jerky displacements to lengthy, smooth ones. Tfp also participate in cell-cell interactions, pathogenesis, biofilm formation, natural DNA uptake, auto-aggregation of cells and development. What is the means by which Tfp bring about the movement of cells?