Defects in contact-stimulated gliding during aggregation by Myxococcus xanthus

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.

Gliding mutants of Myxococcus xanthus with high reversal frequencies and small displacements

Myxococcus xanthus cells move on a solid surface by gliding motility. Several genes required for gliding motility have been identified, including those of the A- and S-motility systems as well as the mgl and frz genes. However, the cellular defects in gliding movement in many of these mutants were unknown. We conducted quantitative, high-resolution single-cell motility assays and found that mutants defective in mglAB or in cglB, an A-motility gene, reversed the direction of gliding at frequencies which were more than 1 order of magnitude higher than that of wild type cells (2.9 min-1 for DeltamglAB mutants and 2.7 min-1 for cglB mutants, compared to 0.17 min-1 for wild-type cells). The average gliding speed of DeltamglAB mutant cells was 40% of that of wild-type cells (on average 1.9 micrometers/min for DeltamglAB mutants, compared to 4.4 micrometers/min for wild-type cells). The mglA-dependent reversals and gliding speeds were dependent on the level of intracellular MglA protein: mglB mutant cells, which contain only 15 to 20% of the wild-type level of MglA protein, glided with an average reversal frequency of about 1.8 min-1 and an average speed of 2.6 micrometers/min. These values range between those exhibited by wild-type cells and by DeltamglAB mutant cells. Epistasis analysis of frz mutants, which are defective in aggregation and in single-cell reversals, showed that a frzD mutation, but not a frzE mutation, partially suppressed the mglA phenotype. In contrast to mgl mutants, cglB mutant cells were able to move with wild-type speeds only when in close proximity to each other. However, under those conditions, these mutant cells were found to glide less often with those speeds. By analyzing double mutants, the high reversing movements and gliding speeds of cglB cells were found to be strictly dependent on type IV pili, encoded by S-motility genes, whereas the high-reversal pattern of mglAB cells was only partially reduced by a pilR mutation. These results suggest that the MglA protein is required for both control of reversal frequency and gliding speed and that in the absence of A motility, type IV pilus-dependent cell movement includes reversals at high frequency. Furthermore, mglAB mutants behave as if they were severely defective in A motility but only partially defective in S motility.

A physical and genetic map of the Stigmatella aurantiaca DW4/3.1 chromosome

A physical map of the myxobacterium Stigmatella aurantiaca DW4/3.1 chromosome was constructed by pulsed-field gel (PFG) long-range mapping. One-and two-dimensional pulsed-field gel analyses were used together with reciprocal double-restriction, cross-hybridization and hybridization fingerprint analysis. These PFG results were confirmed by Smith-Birnstiel analysis, by Southern hybridization using linking clones and clones of a lambda genomic library for the determination of adjacent restriction fragments and by transposon insertion mapping using defined genomic sequences for hybridization. It was thus possible to construct a circular restriction map of the single 9.35 Mbp chromosome of S. aurantiaca based on the endonucleases Asel and Spel. Genetic loci as well as the replication origin were located on the physical map by Southern hybridization using heterologous (derived from Myxococcus xanthus, Escherichia coli and Streptomyces lividans) and homologous probes that are mainly involved in development and cell motility.

Mutations in two new loci that impair both extracellular protein production and development in Myxococcus xanthus

Two transposon insertion mutants of Myxococcus xanthus altered in the secretion of protein as determined by the hydrolytic activities of several enzymes during vegetative growth were also unable to complete fruiting body formation and were severely impaired in sporulation. The insertions were located in the same part of the M. xanthus chromosome but were unlinked by transduction and therefore define two distinct loci, called excA and excB. Since both Exc +/- mutants were able to rescue development of an asgB mutation, they do not belong to the Asg- group, despite of the fact that asg mutants are also Exc +/-. Our results sustain the hypothesis of a possible relationship between protein secretion during vegetative growth and development or sporulation.

Effect of mechanical removal of pili on gliding motility of Myxococcus xanthus

Gliding motility of Myxococcus xanthus is governed by both the adventurous (A) and the social (S) motility gene systems. The presence of pili has previously been shown to be correlated with a genetically intact S-motility system (D. Kaiser, Proc. Natl. Acad. Sci. USA 76:5952-5956, 1979). The purpose of the present work was to study the direct effect of mechanical removal of pill on the social motility of M. xanthus. Depiliation resulted in (i) a loss of streaming motility of A- S+ mutants, i.e., strains which are able to move by virtue of the S-motility system only, (ii) no effect on motility in A+ S- mutants, i.e., strains capable of movement by the A-motility system only, and (iii) a retardation of streaming speed in the wild-type strain (A+ S+). Cell-cell cohesion, another characteristic of social behavior, was not affected by mechanical removal of pill. The observation that mechanical depiliation perturbed the motility of strains which rely on the S-motility system strongly supports a role for pili in social motility of M. xanthus.

Cell-cell interactions that direct fruiting body development in Myxococcus xanthus

The soil bacterium, Myxococcus xanthus initiates a developmental program when nutrients are limited. This results in the formation of a multicellular fruiting body structure filled with differentiated, environmentally resistant spores. At least four cell-cell signals, cell motility, and aggregation functions are required for the completion of fruiting body formation.

Physical map of the Myxococcus xanthus chromosome

The genome of Myxococcus xanthus, which is 9,454 kbp, is one of the largest bacterial genomes. The organization of the DNA and the distribution of genes encoding social and developmental behaviors were examined by using pulsed field gel electrophoresis. Intact genomic DNA was digested with AseI into 16 restriction fragments, which were separated by contour-clamped homogeneous electric field electrophoresis, purified, and radiolabeled. Each AseI fragment was hybridized to SpeI-digested DNA and to an M. xanthus genomic library contained in yeast artificial chromosomes. Some SpeI restriction fragments and yeast artificial chromosome clones contained AseI sites and hybridized with two different AseI restriction fragments, providing evidence for the juxtaposition of these AseI restriction fragments in the chromosome. The deduced AseI physical map is circular, suggesting that this bacterium contains a single, circular chromosome. Transposable elements shown by transduction to be in or near genes of interest were located on specific AseI restriction fragments by restriction analysis and Southern hybridization. Most AseI restriction fragments contained genes involved in social and developmental behaviors.