Recombination in the Vicinity of Insertions of Transposon Tn 5 in MYXOCOCCUS XANTHUS

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.

Mapping of Myxococcus xanthus social motility dsp mutations to the dif genes

Myxococcus xanthus dsp and dif mutants have similar phenotypes in that they are deficient in social motility and fruiting body development. We compared the two loci by genetic mapping, complementation with a cosmid clone, DNA sequencing, and gene disruption and found that 16 of the 18 dsp alleles map to the dif genes. Another dsp allele contains a mutation in the sglK gene. About 36.6 kb around the dsp-dif locus was sequenced and annotated, and 50% of the genes are novel.

Myxococcus xanthus dif genes are required for biogenesis of cell surface fibrils essential for social gliding motility

Myxococcus xanthus social (S) gliding motility has been previously reported by us to require the chemotaxis homologues encoded by the dif genes. In addition, two cell surface structures, type IV pili and extracellular matrix fibrils, are also critical to M. xanthus S motility. We have demonstrated here that M. xanthus dif genes are required for the biogenesis of fibrils but not for that of type IV pili. Furthermore, the developmental defects of dif mutants can be partially rescued by the addition of isolated fibril materials. Along with the chemotaxis genes of various swarming bacteria and the pilGHIJ genes of the twitching bacterium Pseudomonas aeruginosa, the M. xanthus dif genes belong to a unique class of bacterial chemotaxis genes or homologues implicated in the biogenesis of structures required for bacterial surface locomotion. Genetic studies indicate that the dif genes are linked to the M. xanthus dsp region, a locus known to be crucial for M. xanthus fibril biogenesis and S gliding.

New clusters of genes required for gliding motility in Myxococcus xanthus

Gliding is the directed movement of cells across surfaces which occurs in the absence of external organelles such as flagella. Gliding of the complex prokaryote, Myxococcus xanthus, results from the action of two independent sets of genes known as the A (adventurous motility) and S (social motility) genes. Strains with mutations in both systems (A-S-) do not spread on agar surfaces because both individual and group movement is abolished. To generate regulated, transcriptional fusions with operons including A and S genes, we introduced TN5-lac into A- and S- strains to obtain non-motile A-S::Tn5-lac and A::Tn5-lac S- double mutants. These insertions identify five separate clusters of A genes and three separate clusters of S genes on the M. xanthus genome. Some Tn5-lac insertions map near two of the five previously identified motility gene clusters, but at least five new clusters were identified in this search. Single mutations at only one locus, mglA, block motility; the mglA locus is epistatic to A and S motility genes. A- and S- Tn5-lac insertions were transduced into mgl+ and delta mgl strains. The levels of beta-galactosidase activity produced from each A- or S- Tn5-lac insertion are similar in otherwise isogenic mgl+ and delta mgl strains, showing that MglA does not regulate the transcription of many A and S genes.

Mutation and mapping of genes involved in production of the antibiotic TA in Myxococcus xanthus

Transposition of TnV and Tn5lac into Myxococcus xanthus yielded 8,381 kanamycin-resistant mutants that were tested for antibiotic TA production. Twenty-four of the mutants were nonproducers of TA (less than 0.4 ng/ml), and 3 produced a higher level (2.5 micrograms/ml) than the parent strain (1.5 micrograms/ml). For most of the strains, there was 100% cotransduction between kanamycin resistance and the altered TA phenotype. Southern blot analysis of restriction digests of the mutant DNA indicated that the transposons were inserted at different sites on the M. xanthus chromosome. The TA genes were mapped by cotransduction between pairs of mutants following replacement of the initial insert of one of the pair with the tetracycline resistance transposon Tn5-132. Nine of the 13 nonproducers tested were linked over a 36-kb stretch of the chromosome. There was no linkage between one of the overproducers and any of the nonproducers tested.

Low-temperature induction of Myxococcus xanthus developmental gene expression in wild-type and csgA suppressor cells

The csgA gene encodes an extracellular protein that plays an essential role in the regulation of fruiting-body formation and sporulation of Myxococcus xanthus. The csgA suppressor allele soc-500 (formerly referred to as csp-500) was selected based on its ability to restore sporulation to csgA cells under developmental conditions at 32 degrees C. The soc-500 allele was subsequently found to induce sporulation of csgA+ or csgA cells simply by shifting the temperature of vegetatively growing cells to 15 degrees C. Low-temperature-induced sporulation of soc-500 strains occurred in the absence of two requirements for fruiting-body sporulation: low nutrient levels and a high temperature. Low temperature alone caused the expression of many developmentally regulated genes but did not support the development of wild-type cells. The soc-500 allele appears to activate genes involved with sensing nutritional stress. At low temperature on a nutritionally rich medium, soc-500 induced expression of the tps gene which is normally expressed following nutritional shiftdown. The soc-500 allele was cloned and integrated into the wild-type chromosome by site-specific recombination. It was dominant over the wild-type allele in merodiploids and is contained on a 3-kbp DraI-ClaI restriction fragment. The soc-500 transcriptional unit spans a 300-bp PstI-PstI restriction fragment, since deletion of the PstI restriction fragment inhibits both csgA suppression and low-temperature induction. These results suggest that the soc-500 mutation lies in a gene that is involved in nutrient sensing.

Transposon tagging of genes for cell-cell interactions in Myxococcus xanthus

The prokaryote Myxococcus xanthus is a model for cell interactions important in multicellular behavior. We used the transposon TnphoA to specifically identify genes for cell-surface factors involved in cell interactions. From a library of 10,700 insertions of TnphoA, we isolated 36 that produced alkaline phosphatase activity. Three TnphoA insertions tagged cell motility genes, called cgl, which control the adventurous movement of cells. The products of the tagged cgl genes could function in trans upon other cells and were localized primarily in the cell envelope and extracellular space, consistent with TnphoA tagging genes for extracellular factors controlling motility.

asgB, a gene required early for developmental signalling, aggregation, and sporulation of Myxococcus xanthus

The asgB genetic locus of Myxococcus xanthus specifies a function which is required early in the developmental pathway leading to aggregation and sporulation in fruiting bodies. The developmental defect of asgB mutants can be compensated by extracellular complementation using either intact wild-type cells or cell-free supernatants conditioned by developing wild-type cells. A Tn5 insertion was isolated closely linked to asgB480 and facilitated the cloning of both the wild-type (asgB+) and the mutant (asgB480) alleles in Escherichia coli plasmid. Tandem duplications of the asgB locus were constructed in M. xanthus; the completely wild-type phenotype of asgB+/asgB480 partial diploids implies that the asgB480 allele is recessive. This finding, along with extracellular complementation by wild-type cells, is consistent with the hypothesis that the asgB+ locus is required to produce a substance with an intercellular signalling function. At least part of the asgB gene was found to lie within a 1.2 kb SmaI DNA fragment. This 1.2 kb fragment, as well as smaller fragments derived from it, were used as DNA probes in RNA/DNA hybrid analyses of transcription in the asgB region. Two small mRNA species were detected, one about 650 bp long, and the other about 500 bp; the two species of mRNAs apparently overlap. Both mRNAs are present in low, but approximately equal amounts, in vegetatively growing cells. This is consistent with the observation that asg mutants display a mutant vegetative phenotype (a change in colony color and spreading behavior) as well as defective development.

Control of morphogenesis in myxobacteria

The myxobacteria are Gram-negative soil bacteria that live in large communities known as swarms. The most remarkable characteristic of myxobacteria is their ability to form fruiting bodies that have a species-specific shape and color. Fruiting body formation requires the concerted effort of hundreds of thousands of cells. Development is initiated only when two conditions are satisfied. The cells must be nutritionally deprived (environmental signal) and there must be many other cells in the vicinity (intercellular signal). The development of one species, Myxococcus xanthus, has been studied in the most detail. M. xanthus uses amino acids as its primary carbon, nitrogen, and energy source. Starvation for a single amino acid, or for inorganic phosphate, serves as the environmental signal. A variety of intercellular signals appear to control the initiation of development and the timing of subsequent developmental events.

Intercellular signaling is required for developmental gene expression in Myxococcus xanthus

Certain developmental mutants of Myxococcus xanthus can be complemented (extracellularly) by wild-type cells. Insertions of Tn5 lac (a transposon which couples beta-galactosidase expression to exogenous promoters) into developmentally regulated genes were used to investigate extracellular complementation of the A group mutations. A- mutations reduced developmental beta-galactosidase expression from 18 of 21 Tn5 lac insertions tested and that expression was restored to A- Tn5 lac cells by adding wild-type cells. The earliest A-dependent Tn5 lac normally expresses beta-galactosidase at 1.5 hr of development indicating a developmental block at 1-2 hr in A- mutants. A substance which can rescue the expression of this early Tn5 lac is released by wild-type (A+) but not by A- cells. This substance appears in a cell-free wash of wild-type cells or in starvation buffer conditioned by wild-type cells 1-2 hr after development is initiated. The conditioned starvation buffer also restores normal morphological development to an A- mutant.

Role of cell cohesion in Myxococcus xanthus fruiting body formation

Dsp mutants of Myxococcus xanthus have a complex phenotype with abnormal cell cohesion, social motility, and development. All three defects are the result of a single mutation in the dsp locus, a region of DNA about 14 kilobases long. Cohesion appears to play a central role in social motility, since nonsocial mutants exhibit weak agglutination or, in the case of Dsp cells, no agglutination (L. J. Shimkets, J. Bacteriol. 166:837-841, 1986). However, Dsp cells can be agglutinated by cohesive strains of M. xanthus. This provided the opportunity to examine the role of cohesion during development by comparing the developmental phenotype of Dsp cells with that of Dsp cells mixed with cohesive strains. Dsp mutants were unable to complete any of the developmental behaviors: aggregation, fruiting body formation, developmental autolysis, and sporulation. Contact with cohesive strains seemed to restore some developmental characteristics to the Dsp cells. When allowed to develop with wild-type cells, Dsp cells accumulated in fruiting bodies and underwent developmental autolysis, but did not form a significant portion of the spore population. Igl mutants, which may be similar to the previously described frizzy mutants, are cohesive strains that are unable to form fruiting bodies. Mixing Igl cells with Dsp cells under developmental conditions resulted in fruiting body formation, although the Dsp cells were unable to form significant levels of myxospores. In spite of their inability to sporulate under developmental conditions, Dsp mutants did not appear to be defective in the sporulation process. In fact, they formed normal levels of myxospores in response to the chemical inducer glycerol.

Transposon tagging to detect a latent virus in Myxococcus xanthus

Transposon mutagenesis of the bacterium Myxococcus xanthus with the transposon Tn5 revealed a special class of bacterial mutants that transduced the transposon through culture supernatant fluids. Virus-like particles copurified with transducing activity. Transposon tagging for detecting these virus-like particles may be generally useful in isolating endogenous viral agents capable of transferring genetic information between cells.