Site specific transposition of Tn7 into a Rhizobium meliloti megaplasmid

Mini-Tn7 vectors for stable expression of diguanylate cyclase PleD* in Gram-negative bacteria

Background

The cyclic diguanylate (c-di-GMP) is currently considered an ubiquitous second messenger in bacteria that influences a wide range of cellular processes. One of the methodological approaches to unravel c-di-GMP regulatory networks involves raising the c-di-GMP intracellular levels, e.g. by expressing a diguanylate cyclase (DGC), to provoke phenotypic changes.

Results

We have constructed mini-Tn7 delivery vectors for the integration and stable expression of the pleD* gene encoding a highly active DGC, which can be used to artificially increase the intracellular levels of c-di-GMP in Gram negative bacteria. The functionality of these new vectors has been validated in several plant-interacting α- and γ-proteobacteria. Similarly to vector plasmid-borne pleD*, the genome-borne mini-Tn7pleD* constructs provide significant increases in intracellular c-di-GMP, provoking expected phenotypic changes such as enhanced polysaccharide production, biofilm formation and reduced motility. However, the mini-Tn7pleD* constructs resulted far more stable in the absence of antibiotics than the plasmid-based pleD* constructs. Furthermore, we have also implemented an inducible system to modulate pleD* expression and intracellular c-di-GMP rises “on demand”.

Conclusions

mini-Tn7pleD* constructs are very stable and are maintained during bacterial free-living growth as well as during interaction with eukaryotic hosts, in the absence of selective pressure. This high stability ensures experimental homogeneity in time and space with regard to enhancing c-di-GMP intracellular levels in bacteria of interest.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-015-0521-6) contains supplementary material, which is available to authorized users.

Characterization of cytochromes c550 and c555 from Bradyrhizobium japonicum: cloning, mutagenesis, and sequencing of the c555 gene (cycC)

The major soluble c-type cytochromes in cultured cells of Bradyrhizobium japonicum USDA 110 comprised a CO-reactive c555 (Mr, approximately 15,500) and a non-CO-reactive c550 (Mr, approximately 12,500). Levels of cytochrome per gram of soluble protein in aerobic, anaerobic, and symbiotic cells were 32, 21, and 30 nmol, respectively, for c555 and 31, 44, and 65 nmol, respectively, for c550. The midpoint redox potentials (Em,7) of the purified cytochromes were +236 mV for c555 and +277 mV for c550. The CO reactivity of c555 was pH dependent, with maximal reactivity at pH 10 or greater. Rabbit antiserum was produced against purified c555 and used to screen a B. japonicum USDA 110 genomic DNA expression library in lambda gt11 for a downstream portion of the c555 gene (cycC). This sequence was then used to probe a cosmid library for the entire c555 locus. The nucleotide sequence shows an open reading frame of 149 amino acids, with an apparent signal sequence at the N terminus and a heme-binding site near the C terminus. The deduced amino acid sequence is similar to those of the cytochromes c556 of Rhodopseudomonas palustris and Agrobacterium tumefaciens. The cycC gene was mutagenized by insertion of a kanamycin resistance cassette and homologously recombined into the B. japonicum genome. The resulting mutant made no c555 but made normal amounts of c550. The levels of membrane cytochromes were unaffected. The mutant and wild type exhibited identical phenotypes when used to nodulate plants of soybean (Glycine max L. Merr.), with no significant differences in nodule number, nodule mass, or total amount of N2 fixed.

Nodulating Competitiveness of a Nonmotile Tn 7 Mutant of Bradyrhizobium japonicum in Nonsterile Soil

A nonmotile mutant of Bradyrhizobium japonicum serogroup 127 was generated by Tn 7 mutagenesis and matched with the wild type against a common competitor in studies of soybean nodulation in nonsterile soil. The Tn 7 mutant was very similar to the wild type in growth rate in culture, soybean lectin-binding ability, flagellar morphology, and nodulating capability, but it had a longer lag phase. Competing strains were distributed uniformly in soil in various ratios and at different population densities prior to planting. Mutant and wild type were equally prevalent in the seedling rhizosphere at about the time of nodule initiation, suggesting that motility conferred no advantage in rhizosphere colonization. Nodulation success of the Tn 7 mutant was lower than that of the wild type under all test conditions. Differences were greatest at low soil populations of competitors and much less pronounced at initial populations of 10 7 g −1 . The longer lag phase of the Tn 7 mutant may have contributed to its decreased competitiveness, especially at the higher inoculation levels. The antibiotic and motility markers were stable, and the rifampin resistance derived from the parent did not affect adversely the competitiveness of the Tn 7 mutant. We found motility to be of limited importance to the competitiveness of a strain in normal nonsterile soil, where the significance, if any, of this ability may be in migration at the immediate root surface in soils sparsely populated with rhizobial symbionts.

Recognition of Escherichia coli attTn7 by transposon Tn7: lack of specific sequence requirements at the point of Tn7 insertion

Transposon Tn7 inserts at high frequency into a specific site in the Escherichia coli chromosome called attTn7. We show that the point of Tn7 insertion in attTn7 lies within the transcriptional terminator of the bacterial glmS gene. We have exploited the glmS transcription terminator to isolate mutants with altered sequences at the point of Tn7 insertion and have used these mutants to show that the nucleotide sequence at the point of Tn7 insertion is irrelevant to attTn7 target activity. Thus, the nucleotides which provide attTn7 target activity are distinct from the point of Tn7 insertion. We have also examined the effect of transcription on the capacity of attTn7 to act as a target for Tn7 transposition. Our results suggest that transcription of attTn7 does not modulate its Tn7 target activity.

Sequence requirements of Escherichia coli attTn7, a specific site of transposon Tn7 insertion

Transposon Tn7 transposes at high frequency to a specific site, attTn7, in the Escherichia coli chromosome. We devised a quantitative assay for Tn7 transposition in which Tn7-end derivatives containing the cis-acting transposition sequences of Tn7 transpose from a bacteriophage lambda vector upon infection into cells containing the Tn7-encoded transposition proteins. We used this assay to identify a 68-base-pair DNA segment containing the sequences essential for attTn7 target activity. This segment is positioned asymmetrically with respect to the specific point of Tn7 insertion in attTn7 and lacks obvious homology to the sequences at the ends of Tn7 which participate directly in transposition. We also show that some sequences essential for attTn7 target activity are contained within the protein-coding sequence of a bacterial gene.

Organization of the adenyl cyclase (cya) locus of Rhizobium meliloti

A cya-like gene encoding adenyl cyclase from Rhizobium meliloti was localized to a 0.8-kb PstI-EcoRI fragment by subcloning experiments. Experiments in Escherichia coli 'maxicells' identified a R. meliloti cya gene product of 28 kDa, which is significantly smaller than the corresponding protein from enteric bacteria. A control region for the expression of the cya gene in E. coli was found on an adjacent 2.6-kb BglII-BamHI sequence by insertional mutagenesis with Tn5 and phage MudI (ApR lac). The direction of transcription of the cya gene was also determined using a cya::MudIlac fusion. Promoter activity of this cya::lac fusion was not decreased when glucose was added to the culture. The R. meliloti cya gene is conserved among R. meliloti strains but no homology could be detected to other Rhizobium species or to E. coli in DNA hybridization experiments.

Vectors for transposon mutagenesis of non-enteric bacteria

We have constructed a series of transposon delivery vectors derived from pRK2013. Since pRK2013 has a broad host range transfer system and a ColE1 replicon, it can be transferred to, but not replicated in, many non-enteric gram-negative bacteria. Thus pRK2013 provides an effective mechanism for the transient introduction of a transposon. Delivery vectors containing Tn7 (tmp str), Tn10 (tet), Tn10 HH104 (tet), or Tn5-132 (tet) have been constructed. When transposition in Caulobacter crescentus was examined, both Tn7 and Tn5-132 were found to transpose efficiently. In contrast, although the antibiotic resistances of Tn10 and Tn501 (mer) were expressed in C. crescentus, no transposition was observed with either transposon. However, transposition of Tn10 from the Tn10 vectors did occur in Acinetobacter calcoaceticus, and transposition of Tn501 from pMD100 has been demonstrated in Rhizobium japonicum (Bullerjahn and Benzinger 1984). Thus, transposon-host interactions play an important role in the determination of whether a particular transposon can transpose in a given host. Furthermore, the results with C. crescentus indicate that there must be different requirements for host interactions for Tn10 and Tn501 than for Tn5 and Tn7.

Physical and genetic map of the organomercury resistance (Omr) and inorganic mercury resistance (Hgr) loci of the IncM plasmid R831b

Tn7 insertion mutagenesis has been used to facilitate the generation of a physical (restriction endonuclease) and genetic map of the IncM plasmid, R831b. The only selectable phenotypes carried by this 90-kb conjugative plasmid are resistances to inorganic mercury [Hg(II)] and to organomercury compounds. Mutants in the Hgr locus of R831b complemented previously described mutants in the mer operon of the IncFII plasmid R100, indicating functional homology of the locus in each of these different plasmids. However, the R831b Hgr locus is not notably similar in restriction site pattern to either the mer operon of R100 or the mercury resistance transposon, Tn501. Although the enzymes they encode are co-ordinately regulated, the Omr locus of R831b maps approx. 13.5 kb away from the Hgr locus. Three insertions which affect neither phenotype lie between the Hgr and Omr loci; thus, the loci are separated both physically and genetically. One mutant was obtained which tentatively identifies the position of the Tra locus of R831b as adjacent to the Hgr locus.