Efficient insertional mutagenesis in group A streptococci mediated by Tn917 transposon

The replication-conditional thermosensitive vector pTV1-OK (repATs, Kan^r) harbouring the transposon Tn917 (Em^r) was successfully used to mutagenise a clinical Streptococcus pyogenes isolate (CS101). In the initial studies, conditions were established for electrotransformation of the pTV1-OK vector into CS101. Transformants selected on media containing Kan at 29°C were shown to become Kan^s at 39°C and to carry the transposon-linked Em^r marker. One such transformant was chosen for transposition studies. Upon temperature shift, transposition was achieved with a frequency of approximately 0.01% with a plasmid curing efficiency of close to 100%. Southern blot analysis demonstrated that the majority of mutants contained a single copy of Tn917 and showed no evidence for preferential sites of integration (“hot spots”). Screening of Tn917 libraries of S. pyogenes has led to the identification of mutants lacking either haemolysing or plasminogen activating activity. Mutants defective in each of these activities were identified at a frequency of approximately one in 1000 to 4000 colonies. These findings suggest that the pTV1-OK vector can be used for transposon mutagenesis of S. pyogenes and that this strategy will be valuable for identifying virulence factors and regulatory mechanisms in these bacteria.     

Cloning and expression in minicells of the determinant of resistance to fosfomycin from the transposon Tn2921

The fosfomycin resistance determinant of the transposon Tn2921 has been localized and cloned into the plasmid pBR322. A DNA fragment of 1 kb contains all the information required for full expression of the resistance. The level of resistance was unaffected by the plasmid copy number and by the orientation in which the fragment was cloned. Studies in minicells showed that this 1-kb fragment directed the synthesis of a single polypeptide with a molecular mass of 16 kDa.     

Inversions and deletions generated by a mini-gamma delta (Tn1000) transposon.

Intramolecular transposition by an engineered derivative of the transposon gamma delta (Tn1000) is described. This 1-kb element contains inverted repeats of the 40 bp of the delta end of gamma delta, bracketing a kan gene, but it contains no resolution site. Transposition was analyzed in two plasmids; one contained two contraselectable (conditional lethal) genes (thyA and sacB) adjacent to the mini-gamma delta element in a 13.0-kb pBR322/pUC-based two-component plasmid (a heterodimer), and the other contained a different contraselectable gene (strA [rpsL]) in a 13.2-kb three-component plasmid (a heterotrimer). Selection for loss of function of a single contraselectable gene yielded inversions and deletions. Each inversion plasmid was 1 kb larger than the parent plasmid: it had a second copy of mini-gamma delta inserted in the contraselected gene, with that copy plus the intervening segment inverted, and the 5-bp target site duplicated. Each deletion plasmid was smaller than the parent plasmid and had a deletion that extended from one transposon end into or through the contraselected gene for distances of up to 9.4 kb. The frequencies of deletions versus inversions ending in a single target gene were similar, although overall, deletions outnumbered inversions because deletions, but not inversions, into sites beyond the contraselected gene inactivate it. This work also demonstrates that thyA (which encodes thymidylate synthetase) is a useful contraselectable marker.     

Cloning and characterization of the tetracycline resistance determinant of and several promoters from within the conjugative transposon Tn919

Tn919 is a 15- to 16-kilobase (kb) tetracycline resistance conjugative transposon that was originally isolated from Streptococcus sanguis FC1. The tetracycline resistance determinant (tet) was found on a 4.2-kb HindII fragment by in vitro deletion analysis. This fragment was subcloned to a pWV01 origin capable of directing replication in Escherichia coli, Bacillus subtilis, and Streptococcus lactis, and expression was observed in all three genera. In all cases, expression was weaker when only the 4.2-kb cloned fragment rather than the full transposon was present. The resistance gene is of the streptococcal tetM class and codes for a protein of approximately 70 kilodaltons. The restriction map resembles that of the tetM gene of Tn1545 (P. Martin, P. Trieu-Cuot, and P. Courvalin, Nucleic Acids Res. 14:7047-7058, 1986), which codes for a protein of 72.5 kilodaltons. A number of transposon-derived promoter-bearing fragments were also cloned and sequenced. These closely resemble the consensus sequence of E. coli and B. subtilis promoters. Fusion experiments with a truncated lacZ gene indicate the possibility of an open reading frame for one of the promoters.     

Plasmid pSC101 replication mutants generated by insertion of the transposon Tn1000

A derivative of pSC101, pLC709, was constructed by ligation of the HincII-A fragment of pSC101 to the mini-colEI plasmid pVH51 and to a DNA fragment encoding resistance to the antibiotics streptomycin and spectinomycin. Insertions of the transposon Tn1000 (gamma-delta) into the pSC101 replication region of pLC709 were isolated following cotransfer of the plasmid with the sex factor F. The sites of insertion of the transposon were determined by restriction enzyme analysis and the replication and incompatibility properties of the insertion plasmids and DNA fragments cloned from them were analysed. The insertion mutations defined a locus, inc, of approximately 200 base-pairs that is responsible for pSC101-specific incompatibility. Two mutations adjacent to this region inactivate pSC101 replication but can be complemented in trans by a wild-type pSC101 plasmid, and thus define a trans-acting replication function, rep. The inc locus is within a larger region of some 450 base-pairs that is essential for pSC101 replication and that includes the origin of replication. This 450 base-pair segment can replicate in the presence of a helper plasmid that supplies the rep function in trans.     

Mutagenesis of dimeric plasmids by the transposon gamma delta (Tn1000).

The Escherichia coli F factor mediates conjugal transfer of a plasmid such as pBR322 primarily by replicative transposition of transposon gamma delta (Tn1000) from F to that plasmid to form a cointegrate intermediate. Although resolution of this cointegrate always yields a plasmid containing a single gamma delta insertion, the occasional recovery of transposon-free plasmids after conjugal transfer has led to alternative hypotheses for F mobilization. We show here that gamma delta-free plasmids are found after F-mediated conjugal transfer only when the donor plasmid is a dimer and the recipient is Rec+.     

Use of transposon Tn916 as a genetic marker in the rumen

Streptococcus bovis strain SB3 was genetically marked by conjugal transfer of the tetracycline-resistant transposon, Tn916, from Enterococcus faecalis to Strep. bovis. The transposon was stable in the Strep. bovis chromosome in the presence or absence of tetracycline. Streptococcus bovis : Tn916 was introduced into the rumen of experimental sheep and was maintained for at least 76 d. The population was stable in the presence of a grain-based ration but rapidly declined when sheep were transferred to pasture. On return to the grain-based diet, the Strep. bovis : Tn916 population reappeared. These data demonstrate the potential of this technique in studies of microbial interactions in the rumen.     

Use of the Escherichia coli transposon Tn1000 (gamma delta) to generate mutations in Bacillus subtilis DNA

Plasmid pHM2 contains a Bacillus subtilis spoIIA gene and is able to replicate in both Escherichia coli and B. subtilis. The plasmid was mobilized at low frequency by the E. coli F plasmid. Nearly 30% of the mobilized plasmids contained an insert of Tn1000 (gamma delta). Fourteen of the inserts were in the B. subtilis DNA portion of pHM2. Of these, two adjacent inserts abolished expression of the plasmid spoIIA gene in B. subtilis. From the map positions of flanking inserts that do not abolish spoIIA gene expression, it is estimated that the gene is probably not more than 700 bp long.     

Detection and characterization of Tn2501, a transposon included within the lactose transposon Tn951

The DNA sequence spanning coordinates 9.9 to 16.4 kilobases of the lactose transposon Tn951 ( Cornelis et al., Mol. Gen. Genet. 160:215-224, 1978) constitutes a transposable element by itself. Unlike Tn951 ( Cornelis et al., Mol. Gen. Genet. 184:241-248, 1981), this element, called Tn2501 , transposes in the absence of any other transposon. Transposition of Tn2501 proceeds through transient cointegration and duplicates 5 base pairs of host DNA. Tn2501 is flanked by nearly perfect inverted repeats (44 of 48), related to the inverted repeats of Tn21 ( Zheng et al., Nucleic Acids Res. 9:6265-6278, 1982). Unlike Tn21 , Tn2501 does not confer mercury resistance.     

Tn10 insertional mutations of Bacillus subtilis that block the biosynthesis of bacilysin

Transposon mutagenesis was employed to isolate the gene(s) related with the biosynthesis of dipeptide antibiotic in Bacillus subtilis PY79 (a prototrophic derivative of the standard 168 strain). The blocked mutants were phenotypically selected from the transposon library by bioassay and the complete loss of biosynthetic ability was verified through ESI-mass spectrometry analysis. Four different bacilysin nonproducer mutants (Bac(-)::Tn10(ori-spc)) were isolated from the transposon library. The genes involved in bacilysin biosynthesis were identified as thyA (thymidilate synthetase), ybgG (unknown; similar to homocysteine methyl transferase) and oppA (oligopeptide permease), respectively. The other blocked gene was yvgW (unknown; similar to heavy metal-transporting ATPase); however, backcross studies did not verify its involvement in bacilysin biosynthesis. This gene, on the other hand, appeared to be necessary for efficient sporulation and transformation. Opp involvement was significant as it suggested that bacilysin biosynthesis is under or a component of the quorum sensing pathway which has been shown to be responsible for the establishment of sporulation, competence development and onset of surfactin biosynthesis. For verification, it was necessary to check the involvement of peptide pheromones (PhrA or PhrC) internalized by the Opp system and response regulator ComA as the essential components of this global control. phrA, phrC and comA deleted mutants of PY79 were thus constructed and the latter two genes were shown to be essential for bacilysin biosynthesis.     

Cloning of a lac+, bamHI fragment into transposon Tn3 and transposition of the Tn3[lac] element

By use of recombinant DNA techniques, we have inserted the lac⁺ operon into a transposon (Tn3). We constructed the recombinant in such a way that the essential step in assaying for transposition consisted of screening for bacteria with a thermostable Lac⁺ phenotype. Our results showed that transposition of the Tn3[lac⁺] element occurred and that its frequency was derepressed compared to frequencies reported by others for wild-type Tn3 transposition.     

Promoter mutations affecting divergent transcription in the Tn10 tetracycline resistance determinant

The tetracycline resistance determinant in transposon Tn10 consists of two genes, the tetA resistance gene and the tetR repressor gene, that are transcribed from divergent overlapping promoters. We determined the levels of pulse-labeled tet messenger RNA in Escherichia coli strains with the Tn10 tet genes on a multicopy plasmid. Addition of the inducer 5a,6-anhydrotetracycline results in a 270- to 430-fold increase in tetA mRNA and a 35- to 65-fold increase in tetR mRNA. As judged by the relative molar amounts of tetA and tetR mRNA synthesized under maximally inducing conditions, the tetA promoter (tetPA) is 7 to 11 times more active than the two tetR promoters (tetPR1 and tetPR2) combined. We characterized ten mutations in tetPA, including nine single-base-pair substitutions and a 30-base-pair deletion. All of the single-base-pair changes reduce the agreement with the consensus sequence for promoters recognized by E. coli RNA polymerase. Mutations in highly conserved nucleotides result in a 200- to 600-fold reduction in tetPA activity in vivo. Unexpectedly, tetPA mutations reduce by two- to fourfold the combined activity in vivo of tetPR1 and tetPR2, in spite of their locations outside the -35 and -10 regions of tetPR1 and tetPR2. For two tetPA mutations, the negative effect on tetPR activity was also demonstrated in tetR- tetPR-lacZ operon fusion strains, thus eliminating the possibility that it is due to variations in either plasmid copy-number or induction efficiency. The pleiotropic effects of tetPA mutations are discussed in terms of the expectation that the overlapping tet promoters compete for RNA polymerase.     

Crystal structures of the transposon Tn5-carried bleomycin resistance determinant uncomplexed and complexed with bleomycin

The transposon Tn5 carries a gene designated ble that confers resistance to bleomycin (Bm). In this study, we determined the x-ray crystal structures of the ble gene product, designated BLMT, uncomplexed and complexed with Bm at 1.7 and 2.5 A resolution, respectively. The structure of BLMT is a dimer with two Bm-binding pockets composed of two large concavities and two long grooves. This crystal structure of BLMT complexed with Bm gives a precise mode for binding of the antibiotic to BLMT. The conformational change of BLMT generated by binding to Bm occurs at a beta-turn composed of the residues from Gln(97) to Thr(102). Crystallographic analysis of Bm bound to BLMT shows that two thiazolium rings of the bithiazole moiety are in the trans conformation. The axial ligand, which binds a metal ion, seems to be the primary amine in the beta-aminoalanine moiety. This report, which is the first with regard to the x-ray crystal structure of Bm, shows that the bithiazole moiety of Bm is far from the metal-binding domain. That is, Bm complexed with BLMT takes a more extended form than the drug complexed with DNA.     

Overexpression of MerT,the mercuric ion transport protein of transposon Tn501, and genetic selection of mercury hypersensitivity mutations

The small (116 amino acids) inner membrane protein MerT encoded by the transposon Tn501 has been overexpressed under the control of the bacteriophage T7 expression system. Random mutants of MerT were made and screened for loss of mercuric ion hypersensitivity. Several mutantmerT genes were selected and sequenced: Cys24Arg and Cys25Tyr mutations abolish mercury resistance, as do charge-substitution mutations in the first predicted transmembrane helix (Glyl4Arg, Glyl5Arg, Gly27Arg, Ala18Asp), and the termination mutations Trp66Ter and Cys82Ter.     

Transfer of the drug-resistance transposon Tn5 toErwinia herbicola and the induction of insertion mutations

Translocatable drug-resistance element (transposon) Tn5 was transferred through conjugation from its carrier suicidal plasmid pJB4JI, harbored byEscherichia coli, toErwinia herbicola. The frequencies of transfer ranged from 0.4×10^–8 to 26×10^–8 per recipient cell. Membrane filter mating yielded more transconjugants than the spread plate technique. Several insertion mutations resulting in loss of bacteriocinogenicity were detected. The location of Tn5 in the mutant genomes was determined by Southern blot hybridization using Tn5-containing pRZ102 as the³²P-labelled probe. The resulting autoradiogram showed specific hybridization with a 96 megadalton plasmid in nine mutants ofErwinia herbicola out of ten tested. However, in one mutant, the 96 megadalton plasmid was missing; instead a larger plasmid containing Tn5 was apparent which was not present in the original strain. This plasmid may have arisen by dimerization of the 96 megadalton plasmid as a result of Tn5 insertion. Using this method, we show that the insertion of Tn5 in that plasmid may have caused the loss of bacteriocinogenicity. The potential usefulness of this technique in genetic analyses of otherErwinia species and other phytobacteria is discussed.     

Nucleotide sequence of the tetM tetracycline resistance determinant of the streptococcal conjugative shuttle transposon Tn1545.

The nucleotide sequence of the tetracycline resistance gene tetM encoded by streptococcal conjugative shuttle transposon Tn1545 has been determined. The resistance gene was identified as a coding sequence of 1917 base pairs corresponding to a protein with a Mr of 72,500 daltons. This value is in good agreement with that, 68,000 daltons, estimated by SDS-polyacrylamide gel electrophoresis of Escherichia coli minicell extracts. The tetM gene product does not exhibit any sequence homology with either the Gram-negative (tetA, tetB and tetC), or the Bacillus and Staphylococcus tetracycline resistance proteins. The average hydropathy value of the tetM gene product (-0.21) contrasts with those calculated for the other TET proteins which are markedly hydrophobic (0.76 to 0.93). Hybridization experiments performed with an intragenic tetM probe do not support the claim [Taylor, D. (1986), J. Bact. 165, 1037-1039)] that tetracycline resistance in Campylobacter is due to acquisition of tetM.     

Genetic analysis of the pAD1 pheromone response in Streptococcus faecalis, using transposon Tn917 as an insertional mutagen

The conjugative plasmid pAD1 (56.7 kilobases) in Streptococcus faecalis has been shown to confer a mating response to the sex pheromone cAD1 excreted by recipient strains. The response is characterized by the synthesis of a proteinaceous adhesin which coats the surface of the pAD1 -containing donor cell and facilitates the formation of mating aggregates. Donors exposed to cAD1 -containing filtrates of recipients undergo self-aggregation (clumping), an event believed to be associated with an interaction between the adhesin and a binding substance always present on the surface of both recipients and donors. To analyze the molecular processes involved in the mating response, mutants were generated by the erythromycin resistance transposon Tn917 . Transpositions to pAD1 in S. faecalis DS16 gave rise to a number of derivatives that exhibited "constitutive clumping" and the ability to transfer at high frequencies in short (10-min) matings. These mutants fell into two subclasses, which exhibited colony morphologies that were "dry" or "normal". The Tn917 insertions were mapped by restriction enzyme analysis to two separate clusters, designated traA and traB. The dry colony subclass corresponded to traA and represented a span of 1.5 kilobases, whereas the normal subclass corresponded to traB and spanned 1.3 kilobases. The two clusters were separated by 1.7 kilobases in which insertions of Tn917 did not affect the ability to respond normally to cAD1 . Neither type of constitutive clumper produced cAD1 . Another series of insertions exhibited reduced donor potential. In two cases, the reduction in transfer was three to four orders of magnitude; these mapped in traA . In two other cases, the reduction was one to two orders of magnitude. These mapped outside of traA and traB, and one was associated with an increase in plasmid copy number.