Coliphage P1-mediated transduction of cloned DNA from Escherichia coli to Myxococcus xanthus: use for complementation and recombinational analyses

Efficient traceless modification of the P1 bacteriophage genome through homologous recombination with enrichment in double recombinants: A new perspective on the functional annotation of uncharacterized phage genes

The advent of high-throughput omic technologies has caused unprecedented progress in research on bacteriophages, the most abundant and still the least explored entities on earth. Despite the growing number of phage genomes sequenced and the rejuvenation of interest in phage therapy, the progress in the functional analysis of phage genes is slow. Simple and efficient techniques of phage genome targeted mutagenesis that would allow one to knock out particular genes precisely without polar effects in order to study the effect of these knock-outs on phage functions are lacking. Even in the case of model phages, the functions of approximately half of their genes are unknown. P1 is an enterobacterial temperate myophage of clinical significance, which lysogenizes cells as a plasmid. It has a long history of studies, serves as a model in basic research, is a gene transfer vector, and is a source of genetic tools. Its gene products have structural homologs in several other phages. In this perspective article, we describe a simple and efficient procedure of traceless P1 genome modification that could also serve to acquire targeted mutations in the genomes of certain other temperate phages and speed up functional annotations of phage genes.

Bacteriophages vehiculate a high amount of antibiotic resistance determinants of bacterial origin in the Orne River ecosystem

Aquatic environments are important dissemination routes of antibiotic resistance genes (ARGs) from and to pathogenic bacteria. Nevertheless, in these complex matrices, identifying and characterizing the driving microbial actors and ARG dissemination mechanisms they are involved in remain difficult. We here explored the distribution/compartmentalization of a panel of ARGs and mobile genetic elements (MGEs) in bacteria and bacteriophages collected in the water, suspended material and surface sediments from the Orne River ecosystem (France). By using a new bacteriophage DNA extraction method, we showed that, when packaging bacterial DNA, bacteriophages rather encapsidate both ARGs and MGEs than 16S rRNA genes, i.e. chromosomal fragments. We also show that the bacteria and bacteriophage capsid contents in ARGs/MGEs were similarly influenced by seasonality but that the distribution of ARGs/MGEs between the river physical compartments (water vs. suspended mater vs. sediment) is more impacted when these markers were carried by bacteria. These demonstrations will likely modify our understanding of the formation and fate of transducing viral particles in the environment. Consequently, they will also likely modify our estimations of the relative frequencies of the different horizontal gene transfer mechanisms in disseminating antibiotic resistance by reinforcing the roles played by environmental bacteriophages and transduction.

Genetic Modification of Sodalis Species by DNA Transduction

Bacteriophages (phages) are ubiquitous in nature. These viruses play a number of central roles in microbial ecology and evolution by, for instance, promoting horizontal gene transfer (HGT) among bacterial species. The ability of phages to mediate HGT through transduction has been widely exploited as an experimental tool for the genetic study of bacteria. As such, bacteriophage P1 represents a prototypical generalized transducing phage with a broad host range that has been extensively employed in the genetic manipulation of Escherichia coli and a number of other model bacterial species. Here we demonstrate that P1 is capable of infecting, lysogenizing, and promoting transduction in members of the bacterial genus Sodalis, including the maternally inherited insect endosymbiont Sodalis glossinidius. While establishing new tools for the genetic study of these bacterial species, our results suggest that P1 may be used to deliver DNA to many Gram-negative endosymbionts in their insect host, thereby circumventing a culturing requirement to genetically manipulate these organisms.

IMPORTANCE A large number of economically important insects maintain intimate associations with maternally inherited endosymbiotic bacteria. Due to the inherent nature of these associations, insect endosymbionts cannot be usually isolated in pure culture or genetically manipulated. Here we use a broad-host-range bacteriophage to deliver exogenous DNA to an insect endosymbiont and a closely related free-living species. Our results suggest that broad-host-range bacteriophages can be used to genetically alter insect endosymbionts in their insect host and, as a result, bypass a culturing requirement to genetically alter these bacteria.

Producing Gene Deletions in Escherichia coli by P1 Transduction with Excisable Antibiotic Resistance Cassettes

A first approach to study the function of an unknown gene in bacteria is to create a knock-out of this gene. Here, we describe a robust and fast protocol for transferring gene deletion mutations from one Escherichia coli strain to another by using generalized transduction with the bacteriophage P1. This method requires that the mutation be selectable (e.g., based on gene disruptions using antibiotic cassette insertions). Such antibiotic cassettes can be mobilized from a donor strain and introduced into a recipient strain of interest to quickly and easily generate a gene deletion mutant. The antibiotic cassette can be designed to include flippase recognition sites that allow the excision of the cassette by a site-specific recombinase to produce a clean knock-out with only a ~100-base-pair-long scar sequence in the genome. We demonstrate the protocol by knocking out the tamA gene encoding an assembly factor involved in autotransporter biogenesis and test the effect of this knock-out on the biogenesis and function of two trimeric autotransporter adhesins. Though gene deletion by P1 transduction has its limitations, the ease and speed of its implementation make it an attractive alternative to other methods of gene deletion.

High-Frequency Plasmid Transduction by Lactobacillus gasseri Bacteriophage phiadh

The temperate bacteriophage phiadh mediates plasmid DNA transduction in Lactobacillus gasseri ADH at frequencies in the range of 10 to 10 transductants per PFU. BglII-generated DNA fragments from phage phiadh were cloned into the BclI site of the transducible plasmid vector pGK12 (4.4 kb). Phage phiadh lysates induced from Lactobacillus lysogens harboring pGK12 or the recombinant plasmids were used to transduce strain ADH to chloramphenicol resistance. The transduction frequencies of recombinant plasmids were 10- to 10-fold higher than that of native pGK12. The increase in frequency generally correlated with the extent of DNA-DNA homology between plasmid and phage DNAs. The highest transduction frequency was obtained with plasmid pTRK170 (6.6 kb), a pGK12 derivative containing the 1.4- and 0.8-kb BglII DNA fragments of phiadh. DNA hybridization analysis of pTRK170-transducing phage particles revealed that pTRK170 had integrated into the phiadh genome, suggesting that recombination between homologous sequences present in phage and plasmid DNAs was responsible for the formation of high-frequency transducing phage particles. Plasmid DNA analysis of 13 transductants containing pTRK170 showed that each had acquired intact plasmids, indicating that in the process of transduction a further recombination step was involved in the resolution of plasmid DNA monomers from the recombinant pTRK170::phiadh molecule. In addition to strain ADH, pTRK170 could be transduced via phiadh to eight different L. gasseri strains, including the neotype strain, F. Gasser 63 AM (ATCC 33323).

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.

The bcsA gene influences multiple aspects of development in Myxococcus xanthus

M. xanthus strains containing a mutation in the bcsA gene are able to bypass the B and C signaling requirements for development. The bcsA mutant was examined with regards to several aspects of development to better ascertain the function of the bcsA gene. The bcsA mutant developed on nutrient levels sufficient to support vegetative growth in wild-type cells, supporting previous evidence that the bcsA gene inhibits development. The earliest effect of the bcsA mutation on the development program was when cells were beginning to aggregate together to form fruiting bodies. Spores produced by bcsA mutants were hypersusceptible to sodium dodecyl sulfate, suggesting that the bcsA gene is important for optimal spore production. Transcription of the bcsA gene was induced significantly during development at a time when cells were beginning to aggregate together. Collectively, these results indicate that the bcsA gene inhibits development and is also transcriptionally upregulated during development.

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.

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.

Mutants of Myxococcus xanthus dsp defective in fibril binding

The dsp mutant of Myxococcus xanthus lacks extracellular fibrils and as a result is unable to undergo cohesion, group motility, or development (J. W. Arnold and L. J. Shimkets, J. Bacteriol. 170:5765-5770, 1983; J. W. Arnold and L. J. Shimkets, J. Bacteriol. 170:5771-5777, 1983; R. M. Behmlander and M. Dworkin, J. Bacteriol. 173:7810-7821, 1991; L. J. Shimkets, J. Bacteriol. 166:837-841, 1986; L. J. Shimkets, J. Bacteriol. 166:842-848, 1986). However, cohesion and development can be phenotypically restored by the addition of isolated fibrils (R. M. Behmlander, Ph.D. thesis, University of Minnesota, Minneapolis, 1994; B.-Y. Chang and M. Dworkin, J. Bacteriol. 176:7190-7196, 1994). As part of our attempts to examine the interaction of fibrils and cells of M. xanthus, we have isolated a series of secondary mutants of M. xanthus dsp in which cohesion, unlike that of the parent strain, could not be rescued by the addition of isolated fibrils. Cells of M. xanthus dsp were mutagenized either by ethyl methanesulfonate or by Tn5 insertions. Mutagenized cultures were enriched by selection of those cells that could not be rescued, i.e., that failed to cohere in the presence of isolated fibrils. Seven mutants of M. xanthus dsp, designated fbd mutants, were isolated from 6,983 colonies; these represent putative fibril receptor-minus mutants. The fbd mutants, like the parent dsp mutant, still lacked fibrils, but displayed a number of unexpected properties. They regained group motility and the ability to aggregate but not the ability to form mature fruiting bodies. In addition, they partially regained the ability to form myxospores. The fbd mutant was backcrossed into the dsp mutant by Mx4 transduction. Three independently isolated transconjugants showed essentially the same properties as the fbd mutants--loss of fibril rescue of cohesion, partial restoration of myxospore morphogenesis, and restoration of group motility. These results suggest that the physical presence of fibrils is not necessary for group motility, myxospore formation, or the early aggregative stage of development. We propose, however, that the perception of fibril binding is required for normal social behavior and development. The dsp fbd mutants (from here on referred to as fbd mutants) open the possibility of isolating and characterizing a putative fibril receptor gene.

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.

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 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.

The lonD gene is homologous to the lon gene encoding an ATP-dependent protease and is essential for the development of Myxococcus xanthus

Myxococcus xanthus contains two genes (lonV and lonD) homologous to the Escherichia coli lon gene for an ATP-dependent protease. We found that the lonD gene encodes a 90-kDa protein consisting of 827 amino acid residues. The lonD gene product shows 49, 48, and 52% sequence identity to the products of the M. xanthus lonV, E. coli lon, and Bacillus brevis lon genes, respectively. When a lonD-lacZ fusion was used, lonD was expressed during both vegetative growth and development. However, while lonD-disrupted strains were able to grow normally vegetatively, the development of M. xanthus was found to be arrested at an early stage in these strains. The mutant strains were able to form neither fruiting bodies nor myxospores.

Cloning and nucleotide sequence of the Myxococcus xanthus lon gene: indispensability of lon for vegetative growth

The lon gene of Escherichia coli is known to encode protease La, an ATP-dependent protease associated with cellular protein degradation. A lon gene homolog from Myxococcus xanthus, a soil bacterium which differentiates to form fruiting bodies upon nutrient starvation, was cloned and characterized by use of the lon gene of E. coli as a probe. The nucleotide sequence of the M. xanthus lon gene was determined. It contains an open reading frame that encodes a 92-kDa protein consisting of 817 amino acid residues. The deduced amino acid sequence of the M. xanthus lon gene product showed 60 and 56% identity with those of the E. coli and Bacillus brevis lon gene products, respectively. Analysis of an M. xanthus strain carrying a lon-lacZ operon fusion suggested that the lon gene is similarly expressed during vegetative growth and development in M. xanthus. In contrast to that of E. coli, the M. xanthus lon gene was shown to be essential for cell growth, since a null mutant could not be isolated.

Determinants of an unusually stable mRNA in the bacterium Myxococcus xanthus

Myxococcus xanthus is a Gram-negative bacterium which has a complex life cycle that includes development (fruiting body formation). The gene for myxobacterial haemagglutinin, mbhA, is developmentally regulated and highly expressed. In this report we show that the mbhA mRNA is exceptionally stable for a prokaryotic organism, exhibiting a chemical half life (t1/2) of 150 min at 18 h of development. The mbhA mRNA was not stable in vegetatively growing cells nor was it stable when expressed in Escherichia coli. We have used site-directed mutagenesis of the mbhA gene to analyse some of the determinants which mediate the stability of the mbhA transcript. Sequences within the 3'-untranslated region (3'-UTR) were found to be crucial for mRNA stability. This region of mRNA can potentially form an extremely stable stem-loop structure immediately adjacent to the translational stop codon. A deletion within this region caused a 10-fold increase in the decay rate of the transcript. Furthermore, conditions which were associated with reduced mbhA translation or mutations that caused premature termination of translation drastically reduced mRNA stability even in the presence of the wild type 3'-UTR. These results suggest that a significant aspect of mbhA mRNA stability involves a synergistic interaction of the translational machinery with sequence elements within the 3'-UTR.

Methylation of FrzCD, a methyl-accepting taxis protein of Myxococcus xanthus, is correlated with factors affecting cell behavior

Myxococcus xanthus, a nonflagellated gliding bacterium, exhibits multicellular behavior during vegetative growth and fruiting body formation. The frizzy (frz) genes are required to control directed motility for these interactions. The frz genes encode proteins that are homologous to all of the major enteric chemotaxis proteins, with the exception of CheZ. In this study, we characterized FrzCD, a protein which is homologous to the methyl-accepting chemotaxis proteins from the enteric bacteria. FrzCD, unlike the other methyl-accepting chemotaxis proteins, was found to be localized primarily in the cytoplasmic fraction of cells. FrzCD migrates as a ladder of bands on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, reflecting heterogeneity due to methylation or demethylation and to deamidation. FrzCD was shown to be methylated in vivo when cells were exposed to yeast extract or Casitone and demethylated when starved in buffer. We used the methylation state of FrzCD as revealed by Western blot (immunoblot) analyses to search for stimuli that are recognized by the frz signal transduction system. Common amino acids, nucleotides, vitamins, and sugars were not recognized, but certain lipids and alcohols were recognized. For example, the saturated fatty acids capric acid and lauric acid stimulated FrzCD methylation, whereas a variety of other saturated fatty acids did not. Lauryl alcohol and lipoic acid also stimulated methylation, as did phospholipids containing lauric acid. In contrast, several short-chain alcohols, such as isoamyl alcohol, and some other solvents caused demethylation. The relatively high concentrations of the chemicals required for a response may indicate that these chemicals are not the relevant signals recognized by M. xanthus in nature. Isoamyl alcohol and isopropanol also had profound effects on the behavior of wild-type cells, causing them to reverse continuously. Cells of frzB, frzF, and frzG mutants also reversed continuously in the presence of isoamyl alcohol, whereas cells of frzA, frzCD, or frzE mutants did not. On the basis of the data presented, we propose a model for the frz signal transduction pathway in M. xanthus.

Targeted disruption of the Myxococcus xanthus orotidine 5'-monophosphate decarboxylase gene: effects on growth and fruiting-body development

The Myxococcus xanthus gene coding for orotidine 5'-monophosphate (OMP) decarboxylase (EC 4.1.1.23) was cloned. The M. xanthus uraA gene efficiently complemented an Escherichia coli OMP decarboxylase mutant, permitting it to grow in the absence of uracil. Electroporation of M. xanthus with a circular plasmid carrying a selectable uraA::kan gene disruption resulted in homologous recombination at the chromosomal uraA locus. Chromosomal integration of the gene disruption plasmid created heterozygous (uraA+/uraA::kan) tandem duplications. These tandem duplications were unstable and segregated auxotrophic uraA::kan daughters at frequencies of 2 x 10(-4) to 8 x 10(-4) per viable cell. Rare uraA::kan segregants were easily obtained by selecting for resistance to the toxic analog 5-fluoroorotic acid. Our experiments suggest that the cloned uraA gene could facilitate the use of gene duplications in the genetic analysis of M. xanthus development. The uraA mutants could utilize uracil, uridine, or uridine 5'-phosphate for growth, indicating that M. xanthus has pyrimidine salvage pathways. During multicellular development, uraA::kan gene disruption mutants sporulated to wild-type levels but formed smaller and more numerous aggregates than did their uraA+ parent, regardless of whether uracil was added to the medium. Pyrimidine deprivation of uraA mutants, under conditions that otherwise supported vegetative growth, failed to induce fruiting-body development or sporulation.

Transcription of the myxobacterial hemagglutinin gene is mediated by a sigma 54-like promoter and a cis-acting upstream regulatory region of DNA

Myxobacterial hemagglutinin (MBHA) is a major developmentally induced protein that accumulates during the period of cellular aggregation of the fruiting bacterium Myxococcus xanthus. In this study, DNA sequences mediating the transcriptional regulation of mbhA have been identified. Examination of nucleotide sequences upstream of the start site for mbhA transcription has indicated a region of DNA that bears strong homology to the consensus sequence for promoters recognized by the sigma 54 holoenzyme form of RNA polymerase of Escherichia coli and other eubacteria. Deletion of this sequence completely abolished mbhA transcription. Additionally, a cis-acting DNA element, affecting the efficiency of mbhA transcription, has been mapped within a region of DNA 89 to 276 nucleotides upstream of the sigma 54-like sequence. Transposon insertions, mapping within the cis element, drastically reduced mbhA transcriptional activity. These observations suggest that transcription of mbhA requires a productive interaction between a form of RNA polymerase that recognizes a sigma 54-like sequence and a transcriptional activator that binds to DNA sequences upstream of the mbhA promoter.

Social and developmental biology of the myxobacteria

Myxobacteria are soil bacteria whose unusually social behavior distinguishes them from other groups of procaryotes. Perhaps the most remarkable aspect of their social behavior occurs during development, when tens of thousands of cells aggregate and form a colorful fruiting body. Inside the fruiting body the vegetative cells convert into dormant, resistant myxospores. However, myxobacterial social behavior is not restricted to the developmental cycle, and three other social behaviors have been described. Vegetative cells have a multigene social motility system in which cell-cell contact is essential for gliding in multicellular swarms. Cell growth on protein is cooperative in that the growth rate increases with the cell density. Rippling is a periodic behavior in which the cells align themselves in ridges and move in waves. These social behaviors indicate that myxobacterial colonies are not merely collections of individual cells but are societies in which cell behavior is synchronized by cell-cell interactions. The molecular basis of these social behaviors is becoming clear through the use of a combination of behavioral, biochemical, and genetic experimental approaches.

Social and developmental biology of the myxobacteria

Myxobacteria are soil bacteria whose unusually social behavior distinguishes them from other groups of procaryotes. Perhaps the most remarkable aspect of their social behavior occurs during development, when tens of thousands of cells aggregate and form a colorful fruiting body. Inside the fruiting body the vegetative cells convert into dormant, resistant myxospores. However, myxobacterial social behavior is not restricted to the developmental cycle, and three other social behaviors have been described. Vegetative cells have a multigene social motility system in which cell-cell contact is essential for gliding in multicellular swarms. Cell growth on protein is cooperative in that the growth rate increases with the cell density. Rippling is a periodic behavior in which the cells align themselves in ridges and move in waves. These social behaviors indicate that myxobacterial colonies are not merely collections of individual cells but are societies in which cell behavior is synchronized by cell-cell interactions. The molecular basis of these social behaviors is becoming clear through the use of a combination of behavioral, biochemical, and genetic experimental approaches.

Developmental sensory transduction in Myxococcus xanthus involves methylation and demethylation of FrzCD

Myxococcus xanthus is a bacterium that moves by gliding motility and exhibits multicellular development (fruiting body formation). The frizzy (frz) mutants aggregate aberrantly and therefore fail to form fruiting bodies. Individual frz cells cannot control the frequency at which they reverse direction while gliding. Previously, FrzCD was shown to exhibit significant sequence similarity to the enteric methyl-accepting chemotaxis proteins. In this report, we show that FrzCD is modified by methylation and that frzF encodes the methyltransferase. We also identify a new gene, frzG, whose predicted product is homologous to that of the cheB (methylesterase) gene from Escherichia coli. Thus, although M. xanthus is unflagellated, it appears to have a sensory transduction system which is similar in many of its components to those found in flagellated bacteria.

Genetic analysis of tag mutants of Myxococcus xanthus provides evidence for two developmental aggregation systems

Temperature-dependent aggregation mutants (tag) of the myxobacterium Myxococcus xanthus aggregated into mounds and developed into fruiting bodies normally at 28 degrees C; however, they failed to form mounds at 34 degrees C. The timing of sporulation was unaffected by the mutations, and normal numbers of spores were produced at both permissive and restrictive temperatures. This class of mutations was originally identified through screening of ethyl methanesulfonate (EMS)-generated mutations. Subsequent work identified a linked insertion of transposon Tn5, which was used to map the EMS-generated mutations to four loci. In this paper, we describe the cloning of the tag loci and the use of transposon mutagenesis to further analyze the tag loci. Nine tag complementation groups spanning 8.5 kilobase pairs of DNA were identified through mapping of 28 independent Tn5 insertions. All insertion and deletion mutants had the same phenotype as the EMS mutants: they were temperature sensitive for mound formation. This result suggests that M. xanthus has at least two sets of genes for developmental aggregation. The tag genes constitute one set of these genes; they are required for normal development at 34 degrees C but are not required for normal development at 28 degrees C.

A segment of Myxococcus xanthus ops DNA functions as an upstream activation site for tps gene transcription

A segment of DNA located between 131 and 311 base pairs (bp) upstream from the transcriptional start of the Myxococcus xanthus ops gene (-131 to -311) was shown to function as an upstream activation site (UAS) for developmentally regulated transcription from the tps gene promoter region. The activation of early developmental transcription by the ops UAS was independent of orientation and could be increased by the addition of a second copy of the UAS. The ops UAS segment continued to function when placed 1.5 kbp upstream from the transcription initiation site. DNA from the tps promoter region was required for transcriptional activation by the ops UAS, and a specific requirement for the sequence of tps DNA between -34 and -66 was demonstrated. Several specific ops UAS DNA-protein complexes were observed after incubation of this DNA segment with an extract of early developmental M. xanthus cells. Extracts of vegetative cells contained much less ops UAS-specific DNA-binding activity. When the distance between the tps and ops genes was increased from 2 to 15 kbp by insertion of a transduced segment of DNA, the amount of developmentally induced tps RNA was found to be about one-third that found in wild-type M. xanthus. Our observations suggest that the regulatory region of the ops gene functions not only to control ops gene expression but also to increase early developmental expression of the tps gene located about 2 kbp downstream on the M. xanthus chromosome.

Physical mapping of the Myxococcus xanthus genome by random cloning in yeast artificial chromosomes

Random segments of Myxococcus xanthus DNA were cloned in yeast artificial chromosomes (YACs) to construct a physical map of the genome. EcoRI restriction maps of 409 YAC clones with inserts averaging 111 kilobase pairs (kb) were determined. Comparison to the map of a 300-kb region of M. xanthus obtained from clones in Escherichia coli indicates that segments of DNA cloned in YACs are stably maintained in yeast and that their sequences accurately reflect the structure of the Myxococcus genome. The 409 YAC inserts were ordered within 60 map segments (contigs) by aligning their EcoRI restriction maps and by hybridization with 18 gene-specific DNA probes. These 60 map segments may represent the entire Myxococcus genome and could be used to organize its genetic information. This study illustrates the utility of YACs for cloning large segments of DNA and for reliable long-range genomic mapping.

Evidence that the Myxococcus xanthus frz genes are developmentally regulated

The frizzy (frz) mutants of Myxococcus xanthus are unable to form fruiting bodies. Instead of forming discrete mounds, these strains aggregate as filaments which have a circular and tangled appearance. Mutations leading to this phenotype have been mapped to five complementation groups, frzA, frzB, frzCD, frzE, and frzF. All have been found to be involved in the control of directional movement of the bacteria and, except for frzB, to be homologous to the chemotaxis genes of enteric bacteria. In this report we present a study of the regulation of expression of the first four genes of the frz gene cluster (frzA, frzB, frzCD, and frzE) by using Tn5-lac transcriptional fusions as reporters of gene expression. We found that these frz genes are developmentally regulated, with their transcription peaking at about the time of early mound formation (12 to 18 h). Analysis of FrzCD expression by enzyme-linked immunosorbent assay showed a 10-fold greater induction at 15 h of development over the level of vegetative cell expression. Northern blot hybridization analysis suggested that the frz genes were arranged as an operon. To test this hypothesis, double mutants were constructed which contained Tn5-132 either upstream or downstream of the reporter Tn5-lac. The expression of the frz genes in the double mutants was consistent with the hypothesis that the first four genes (frzA, frzB, frzCD, and frzE) are organized as an operon with an internal promoter. Insertion mutations in frzCD lowered gene expression whether they were upstream or downstream of the reporter Tn5-lac, suggesting that the FrzCD protein regulates transcription of the entire operon from a promoter upstream of frzA. Evidence is presented suggesting that FrzE is required for induction of transcription as well. When frz mutations were placed in strains that were unable to aggregate (tag), the frz genes were expressed at an elevated level on fruiting agar; this high level of expression was maintained for several days. These results suggest that the tag gene products interact with the frz functions.

dsg, a gene required for Myxococcus development, is necessary for cell viability

Previous work identified the dsg gene as necessary for cell-cell interaction in Myxococcus xanthus. Point mutations of this gene, such as dsg-439, are viable, but insertions of Tn5 within the dsg gene (dsg::Tn5) are lethal. Partial diploids, dsg::Tn5/dsg+ or dsg::Tn5/dsg-429 or dsg::Tn5/dsg-439, are also viable, showing that the lethal effect of the haploid insertions is due to loss of function. Thus the evidence implies that the dsg gene is essential for viability as well as development, but its essential quality differs between growth and development because dsg-429 and dsg-439 mutants grow normally, but are unable to develop.

dsg, a gene required for cell-cell interaction early in Myxococcus development

dsg mutants of Myxococcus xanthus are conditionally defective in fruiting body development, including sporulation. Unable to develop on their own, these mutants can assemble fruiting bodies with spores if they are mixed with wild-type cells. To elucidate the developmental defect in dsg mutants by close comparison with wild type, such mutants have been backcrossed by transduction, using a closely linked insertion of transposon Tn5 for selection. Backcrossed dsg mutants form aggregates that are larger, less compact, and less symmetrical than dsg+ fruiting bodies. Also, the starvation-induced sporulation in dsg aggregates is delayed and reduced. However, dsg mutants can be induced by glycerol or dimethyl sulfoxide to sporulate at levels approaching those of wild type. dsg mutants may thus have a primary defect early in development which diminishes their capacity to aggregate and which indirectly decreases the number of fruiting body spores. The linked insertion of Tn5 also facilitated cloning the dsg gene. The cloned dsg+ allele was shown to be dominant to both the dsg-429 and dsg-439 alleles, and both mutant alleles were shown to belong to the same genetic complementation group. Subcloning of restriction fragments, deletions, and insertions of transposon Tn5 agree in locating the dsg gene to an 850-base-pair segment of the cloned region.

Gliding motility in Myxococcus xanthus: mgl locus, RNA, and predicted protein products

Mutants of Myxococcus xanthus that had lost the ability to glide were examined to elucidate the mechanism of gliding motility. Nonmotile mutants resulting from a single mutational step were all defective at the same locus, mgl, which implied an important role for the mgl product(s) in gliding. Deletion experiments, transposon insertion mutagenesis, and genetic rescue of mgl mutants mapped the locus to a 1.6-kilobase segment of Myxococcus DNA. Two species of RNA that hybridized with mgl DNA were found both during vegetative growth and during the starvation-induced development of fruiting bodies, which also requires cell movement. The two RNA species, of 1.5 and 1.3 kilobases, had the same 5' to 3' orientation and overlapped extensively. The DNA sequences of mgl+ and of seven mgl mutants were determined. Each mutant differed from mgl+ by a single-base-pair change in the sequence. Two adjacent open reading frames were found in the sequence hybridizing to both species of mgl RNA. Six of the single-base-pair changes, each of which would result in a single-amino-acid change, and an insertion-produced mgl mutation were located in the downstream open reading frame. This open reading frame (of 195 amino acids) is therefore an mgl gene, called mglA. The function of the upstream open reading frame is not known with certainty, although it does contain one of the mgl mutant sites and could be a second mgl gene.

Site-specific integration and expression of a developmental promoter in Myxococcus xanthus

A series of intercellular signals are involved in the regulation of gene expression during fruiting body formation of Myxococcus xanthus. Mutations which block cell interactions, such as csgA (formerly known as spoC), also prevent expression of certain developmentally regulated promoters. csgA+ cells containing Tn5 lac omega DK4435, a developmentally regulated promoter fused to lacZ, began synthesizing lacZ mRNA 12 to 18 h into the developmental cycle. beta-Galactosidase specific activity increased about 12 h later. Neither lacZ mRNA nor beta-galactosidase activity was detected in a developing csgA mutant containing omega DK4435. The developmental promoter and its fused lacZ reporter gene were cloned into a pBR322-derived plasmid vector containing a portion of bacteriophage Mx8. These plasmids preferentially integrated into the M. xanthus chromosome by site-specific recombination at the bacteriophage Mx8 attachment site and maintained a copy number of 1 per chromosome. The integrated plasmids were relatively stable, segregating at a frequency of 0.0007% per generation in the absence of selection. The cloned and integrated promoter behaved like the native promoter, expressing beta-galactosidase at the proper time during wild-type development and failing to express the enzyme during development of a csgA mutant. The overall level of beta-galactosidase expression in merodiploid cells containing one native promoter and one promoter fused to lacZ was about half that of cells containing a single promoter fused to lacZ. These results suggest that the timing of developmentally regulated gene expression is largely independent of the location of this gene within the chromosome. Furthermore, they show that site-specific recombination can be a useful tool for establishing assays for promoter or gene function in M. xanthus.

Genetic identification and cloning of a gene required for developmental cell interactions in Myxococcus xanthus

Developmental mutants of Myxococcus xanthus have been previously described which appear to be defective in required cell-cell interactions. These mutants fall into four phenotypic classes, Asg, Bsg, Csg, and Dsg, each of which is unable to differentiate into spores but can be rescued by extracellular complementation by wild-type cells or by mutants of a different class. We report the identification of one of the loci in which mutations result in a Bsg phenotype. The cloned locus was contained on a 12-kilobase EcoRI fragment and then localized by subcloning and a combination of in vitro and transposon mutagenesis. All mutations in this locus behave as a single complementation group, which we designate bsgA (formerly ssbA). Each of the bsgA mutations results in a nonsporulating phenotype, which can be rescued by extracellular complementation. Furthermore, we report that the bsgA mutants have a distinctive interaction with wild-type cells when vegetatively growing, swarming colonies converge.

Tn5-mediated transposition of plasmid DNA after transduction to Myxococcus xanthus

After coliphage P1-mediated transfer of Tn5-containing plasmid DNA from Escherichia coli to Myxococcus xanthus, transductants were identified which contained plasmid sequences integrated at many sites on the bacterial chromosome. The unaltered plasmid DNA sequences in these transductants were apparently flanked by intact Tn5 or IS50 sequences. These results suggest that Tn5-mediated transposition has occurred and provide a method for integrating plasmid DNA into the M. xanthus chromosome without the requirement for homologous recombination.

Localization of the cis-acting regulatory DNA sequences of the Myxococcus xanthus tps and ops genes

The cis-acting regulatory regions of the tps and ops genes of Myxococcus xanthus were localized by analyzing the expression of fusions of these genes with the lacZ gene. A 201-base-pair (bp) fragment of tps DNa extending 95 bp upstream (-95) from the transcriptional start was sufficient to direct developmentally regulated expression of fusion gene activity. The segment of tps DNA between -95 and -81 contained information necessary for developmental regulation. A segment of ops DNa extending upstream to -131 directed a very low level of ops-lacZ fusion expression, but the inclusion of DNA to -208 greatly increased the amount of developmentally regulated expression. M. xanthus DNA upstream from -108 in the tps gene and -311 in the ops gene was required for maximal expression of gene fusion activity. The upstream regulatory regions of both the tps and ops genes seem to be involved in positive transcriptional regulation. Two mutations, a deletion of 1 bp at -8 in the tps gene and a 3-bp substitution at -27 to -29 in the ops gene, greatly increased the level of vegetative expression of gene fusion activity, suggesting that both genes may also be subject to negative regulation in M. xanthus.

Use of recombination techniques to examine the structure of the csg locus of Myxococcus xanthus

The myxobacteria are among the simplest organisms with a developmental cycle that is dependent on cell cooperation, and they provide an outstanding system with which to study genes involved in cell interactions. Myxococcus xanthus cells which acquire a csg mutation (formerly known as spoC) lose three different traits, the ability to sporulate, the ability to stimulate adjacent Csg cells to sporulate, and the ability to ripple. The boundaries of the csg locus were determined by transferring a recombinant DNA molecule containing all or part of the locus to Csg mutants and examining the sporulation and rippling phenotypes of the transductants. Three methods were used to integrate the csg locus into the chromosome. First, the entire molecule was integrated into the chromosome by a single homologous crossover. Second, a portion of the molecule was integrated into the chromosome by two flanking homologous crossovers. Third, the entire molecule was integrated into the chromosome by site-specific recombination at a bacteriophage attachment site. Together, these techniques suggested that all of the functions of the csg locus are carried on a DNA fragment of 1.9 kbp or less. The locus appears to contain two smaller units of function. Transposon insertions or deletions in the right end of the locus disrupted sporulation and intercellular complementation of Csg mutants for sporulation, but did not disrupt rippling. The intercellular complementation of Csg mutants may reflect a natural and necessary step in the sporulation of wild-type cells, since the ability to sporulate and the ability to stimulate Csg mutants to sporulate were inseparable by any of these methods.

Cloning and characterization of the Aeromonas caviae recA gene and construction of an A. caviae recA mutant

A recombinant plasmid carrying the recA gene of Aeromonas caviae was isolated from an A. caviae genomic library by complementation of an Escherichia coli recA mutant. The plasmid restored resistance to both UV irradiation and to the DNA-damaging agent methyl methanesulfonate in the E. coli recA mutant strain. The cloned gene also restored recombination proficiency as measured by the formation of lac+ recombinants from duplicated mutant lacZ genes and by the ability to propagate a strain of phage lambda (red gam) that requires host recombination functions for growth. The approximate location of the recA gene on the cloned DNA fragment was determined by constructing deletions and by the insertion of Tn5, both of which abolished the ability of the recombinant plasmid to complement the E. coli recA strains. A. caviae recA::Tn5 was introduced into A. caviae by P1 transduction. The resulting A. caviae recA mutant strain was considerably more sensitive to UV light than was its parent. Southern hybridization analysis indicated that the A. caviae recA gene has diverged from the recA genes from a variety of gram-negative bacteria, including A. hydrophila and A. sobria. Maxicell labeling experiments revealed that the RecA protein of A. caviae had an Mr of about 39,400.

Mode of insertion of the broad-host-range plasmid RP4 and its derivatives into the chromosome of Myxococcus xanthus

The mode of insertion of the broad-host-range plasmid RP4 into the chromosome of Myxococcus xanthus strain DZ1 has been analyzed. The plasmid integrated in numerous sites of the chromosome and generated insertional mutations. There is a hot spot of integration located between 31.5 and 34.5 kb clockwise from the EcoRI site of the plasmid. In the absence of this segment the insertion can, however, take place, but much less efficiently. The presence of transposable elements on the plasmid decreases severely the insertion frequency. Once integrated, RP4 could be transferred back to Escherichia coli, either by precise excision or with a segment of the Myxococcus chromosome. The role of site-specific recombination in RP4 integration is discussed.

Cloning of the gene for myxobacterial hemagglutinin and isolation and analysis of structural gene mutations

Myxobacterial hemagglutinin (MBHA) is a major developmentally induced protein that accumulates during the period of cellular aggregation in the bacterium Myxococcus xanthus. It has been shown that this lectin is targeted to the cell surface and periplasmic space of developmental cells, suggesting that it may play a role in cell-cell recognition or agglutination. We have cloned the structural gene for MBHA by using synthetic deoxyoligonucleotides containing sequences deduced from the amino acid sequence of MBHA and have used the cloned gene to construct strains of M. xanthus that cannot synthesize MBHA. We found that although the MBHA-deficient strains are delayed in their developmental time course, they are otherwise able to aggregate and sporulate normally. Our results suggest that MBHA may function to increase the efficiency of fruiting-body formation but is not a critical component of cellular aggregation.

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.

A global analysis of developmentally regulated genes in Myxococcus xanthus

Tn5 lac is a transposon that fuses the transcription of lacZ to exogenous promoters. We generated 2374 Tn5 lac insertion-containing strains of Myxococcus xanthus, a soil bacterium that undergoes multicellular development which culminates in the formation of spores. Thirty-six strains were identified that specifically increase beta-galactosidase expression at some particular time during development and these expression times range from minutes after starvation initiates development to 24 hr, when sporulation begins. Different maximum levels of beta-galactosidase expression were also observed and the maximum for many strains that begin beta-galactosidase expression late in development was observed only if spores were disrupted. Seven of the 36 strains display mild to severe defects in aggregation and/or sporulation, as did an additional five strains whose beta-galactosidase expression was not developmentally regulated. Restriction maps of the DNA adjacent to the Tn5 lac insertions that are developmentally regulated and/or cause developmental defects show that most of the 41 insertions are in different regions of the Myxococcus genome. The developmentally regulated Tn5 lac insertions described here provide a set of at least 29 new developmental markers for Myxococcus.

Genetic analysis of Myxococcus xanthus and isolation of gene replacements after transduction under conditions of limited homology

Genetic analysis of Myxococcus xanthus is greatly facilitated by the ability to introduce cloned DNA into M. xanthus to generate gene replacement and merodiploid strains. However, gene replacement strains are difficult to obtain when the region(s) of homology between the cloned DNA and the M. xanthus chromosome is limited (less than 1 kilobase). We found that gene replacements can be obtained at an increased frequency by a two-step procedure involving the use of bacteriophage P1 to isolate merodiploid strains followed by generalized transduction to another M. xanthus strain by using phage Mx4.

Analysis of the products of the Myxococcus xanthus frz genes

The frizzy (frz) genes of Myxococcus xanthus control the ability of cells to reverse direction of gliding motility. The orientation of the frz genes was studied by isolating transcriptional fusions with the transposon derivative Tn5-lac. The frz genes were then cloned in the proper orientation in an expression vector. By using maxicell experiments, we were able to identify several labeled bands which were plasmid encoded. To identify the labeled proteins and their respective genes, we constructed deletion plasmids in which various regions of the insert DNA had been removed. The plasmid-encoded proteins were then labeled in maxicell experiments, and the bands which correspond to the frzCD, frzE, and frzF gene products were identified. The sizes of the gene products agreed with the genetic and physical map of the cloned DNA.

The construction of a cloning vector designed for gene replacement in Bordetella pertussis

We report here the construction of a plasmid cloning vector, pRTP1, designed to facilitate exchange of cloned and chromosomal alleles of the human bacterial pathogen Bordetella pertussis. pRTP1 provides the ability to successively select two homologous recombination events within the cloned sequences. The first is by selection for maintenance of the ampicillin-resistance gene on the plasmid which is unable to replicate autonomously after transfer via conjugation. The second selection, via streptomycin (Sm) selection, is against the maintenance of vector sequences which contain a gene encoding the Sm-sensitive allele of the gene for ribosomal protein S12 thus rendering an otherwise Sm-resistant strain Sm-sensitive. We demonstrate the use of this vector to introduce an unmarked mutation, constructed in vitro, into the chromosomal locus encoding pertussis toxin.

"Frizzy" genes of Myxococcus xanthus are involved in control of frequency of reversal of gliding motility

Myxococcus xanthus, a Gram-negative bacterium, has a complex life cycle that includes fruiting body formation. Frizzy (frz) mutants are unable to aggregate normally, instead forming frizzy filamentous aggregates. We have found that these mutants are defective in the control of cell reversal during gliding motility. Wild-type cells reverse their direction of gliding about every 6.8 min; net movement occurs since the interval between reversals can vary widely. The frzA-C, -E and -F mutants reverse their direction of movement very rarely, about once every 2 hr. These mutants cannot aggregate normally and give rise to frizzy filamentous colonies on fruiting agar or motility agar. In contrast, frzD mutants reverse their direction of movement very frequently, about once every 2.2 min; individual cells show little net movement and form smooth-edged "nonmotile" type colonies. Genetic analysis of the frzD locus shows that mutations in this locus can be dominant to the wild-type allele and that its gene product(s) must interact with the other frz gene products. Our results suggest that the frz genes are part of a system responsible for directed movement of this organism.

Cell interactions in myxobacterial growth and development

During their complex life cycle, myxobacteria manifest a number of cell interactions. These include contact-mediated interactions as well as those mediated by soluble extracellular signals. Some of these interactions are well-defined; in addition, the tools for molecular and genetic analysis of these interactions in Myxococcus xanthus are now available.

Differential expression of protein S genes during Myxococcus xanthus development

Protein S, the most abundant protein synthesized during development of the fruiting bacterium Myxococcus xanthus, is coded by two highly homologous genes called protein S gene 1 (ops) and protein S gene 2 (tps). The expression of these genes was studied with fusions of the protein S genes to the lacZ gene of Escherichia coli. The gene fusions were constructed so that expression of beta-galactosidase activity was dependent on protein S gene regulatory sequences. Both the gene 1-lacZ fusion and the gene 2-lacZ fusion were expressed exclusively during fruiting body formation (development) in M. xanthus. However, distinct patterns of induction of fusion protein activity were observed for the two genes. Gene 2 fusion activity was detected early during development on an agar surface and could also be observed during nutritional downshift in dispersed liquid culture. Gene 1 fusion activity was not detected until much later in development and was not observed after downshift in liquid culture. The time of induction of gene 1 fusion activity was correlated with the onset of sporulation, and most of the activity was spore associated. This gene fusion was expressed during glycerol-induced sporulation when gene 2 fusion activity could not be detected. The protein S genes appear to be members of distinct regulatory classes of developmental genes in M. xanthus.

Cloning and complementation analysis of the "Frizzy" genes of Myxococcus xanthus

Fruiting-body formation in Myxococcus xanthus involves the aggregation of cells into raised mounds, where they sporulate. "Frizzy" mutants fail to aggregate into mounds, but rather aggregate into "frizzy" filaments (D.R. Zusman 1982). The frizzy mutations (frz) were found to be genetically linked. The region of DNA carrying the frz genes was cloned in Escherichia coli by selecting for the kanamycin resistance element present on a transposon Tn5 insertion linked to the frz genes. Phage P1 mediated transduction of the cloned DNA into M. xanthus frizzy mutants showed that the cloned DNA could complement the frz mutations. The cloned DNA was analyzed by isolating and characterizing new Tn5 insertions at short intervals within the M. xanthus DNA and by constructing in vitro deletions. The mutated DNA was then transduced into M. xanthus where the cloned DNA became integrated into the bacterial chromosome as gene replacements or as merodiploids. The gene replacement strains allowed us to define the limits of the frz region, since Tn5 insertions in the frz genes resulted in the frizzy phenotype. The merodiploid strains allowed us to perform complementation analyses. Using appropriate crosses, we were able to identify 5-6 frz complementation groups on 7.5 kb of cloned DNA. One of the complementation groups was separated from the others by 1.4 kb of DNA, whereas the others were contiguous. The different frz loci behave as separate transcriptional groups although interactions between some of the gene products are indicated.

Transfer of plasmid RP4 to Myxococcus xanthus and evidence for its integration into the chromosome

The broad-host-range plasmid RP4 and its derivative R68.45 were transferred to Myxococcus xanthus DK101 and DZ1; RP4 was maintained integrated in the chromosome. Loss of plasmid markers occurred during the growth of the transconjugants, which could be prevented by selective pressure with oxytetracycline. The integrated plasmid was transferred back to Escherichia coli often as RP4-prime plasmids carrying various segments of the M. xanthus chromosome. It also mediated chromosomal transfer between M. xanthus strains.