Plasmid fusions mediated by one end of TnA

The Tn3-family of Replicative Transposons

Transposons of the Tn3 family form a widespread and remarkably homogeneous group of bacterial transposable elements in terms of transposition functions and an extremely versatile system for mediating gene reassortment and genomic plasticity owing to their modular organization. They have made major contributions to antimicrobial drug resistance dissemination or to endowing environmental bacteria with novel catabolic capacities. Here, we discuss the dynamic aspects inherent to the diversity and mosaic structure of Tn3-family transposons and their derivatives. We also provide an overview of current knowledge of the replicative transposition mechanism of the family, emphasizing most recent work aimed at understanding this mechanism at the biochemical level. Previous and recent data are put in perspective with those obtained for other transposable elements to build up a tentative model linking the activities of the Tn3-family transposase protein with the cellular process of DNA replication, suggesting new lines for further investigation. Finally, we summarize our current view of the DNA site-specific recombination mechanisms responsible for converting replicative transposition intermediates into final products, comparing paradigm systems using a serine recombinase with more recently characterized systems that use a tyrosine recombinase.

Different pathways to acquiring resistance genes illustrated by the recent evolution of IncW plasmids

DNA sequence analysis of five IncW plasmids (R388, pSa, R7K, pIE321, and pIE522) demonstrated that they share a considerable portion of their genomes and allowed us to define the IncW backbone. Among these plasmids, the backbone is stable and seems to have diverged recently, since the overall identity among its members is higher than 95%. The only gene in which significant variation was observed was trwA; the changes in the coding sequence correlated with parallel changes in the corresponding TrwA binding sites at oriT, suggesting a functional connection between both sets of changes. The present IncW plasmid diversity is shaped by the acquisition of antibiotic resistance genes as a consequence of the pressure exerted by antibiotic usage. Sequence comparisons pinpointed the insertion events that differentiated the five plasmids analyzed. Of greatest interest is that a single acquisition of a class I integron platform, into which different gene cassettes were later incorporated, gave rise to plasmids R388, pIE522, and pSa, while plasmids R7K and pIE321 do not contain the integron platform and arose in the antibiotic world because of the insertion of several antibiotic resistance transposons.

Transposition of the gene encoding a TEM-12 extended-spectrum beta-lactamase

An isolate of Klebsiella oxytoca from the blood culture of a child with leukemia was found to produce two beta-lactamases, at least one of which conferred resistance to ceftazidime. Genes encoding both enzymes were located on a single self-transmissible 100-kb plasmid, pOZ201. This plasmid was introduced into Escherichia coli UB5201 (pACYC184), and the gene encoding one beta-lactamase was transposed onto plasmid pACYC184 by exploiting a gene dosage effect. The transposable gene was found to encode a TEM-12 enzyme as determined by nucleotide sequencing. This gene was subsequently transposed onto plasmid pUB307. The transposable element encoding the TEM-12 enzyme has been designated Tn841. Both plasmids pACYC184::Tn841 and pUB307::Tn841 were shown to encode a beta-lactamase with the same isoelectric point and substrate profile as the TEM-12 beta-lactamase. Transposon Tn841, at approximately 7 kb, is larger than TnA (4.8 kb) and transposes at a lower frequency. Although it produced a resolvase which can complement the resolvase of Tn3, its transposase function was not able to complement the transposition of a TnA element which lacked transposase. The occurrence of a gene encoding an extended-spectrum beta-lactamase on a transposable element in a clinically significant bacterium is potentially a cause for concern for the spread of resistance to the extended-spectrum cephalosporins.

Molecular characterization of the class II multiresistance transposable element Tn1403 from Pseudomonas aeruginosa

Transposon Tn1403 is a 19.9-kb multiresistance class II transposable element originally found on the RPL11 plasmid from a clinical isolate of Pseudomonas aeruginosa. It encodes resistance to ampicillin (PSE-1 beta-lactamase), streptomycin and spectinomycin (aadA and aphC), and chloramphenicol (cat). It has structural homology with the tnpM and tnpI sequences of Tn21 and inverted repeats and res and tnpR sequences of Tn501, but it has no structural homology nor functional complementation with the resolvase gene of Tn21 or Tn3. Sequence analysis revealed long inverted repeats at each extremity of Tn1403 containing 38-bp inverted repeats that were 97.4% similar to those of Tn1721 and 5-bp direct repeats. Transposition assays showed a low frequency of transposition (3.5 x 10(-6)) compared with that of Tn3 (3.3 x 10(-3)) and no resolution of cointegrates.

Identification and characterization of Tn4653, a transposon covering the toluene transposon Tn4651 on TOL plasmid pWW0

A Pseudomonas TOL plasmid pWW0 possesses toluene degradative pathway (xyl) genes. Unstable maintenance of a pWW0 derivative in Escherichia coli allowed us to identify two transposable elements each carrying all the xyl genes. One element corresponded to a 56 kb transposon, Tn4651, which we had previously characterized. The other element newly identified in this study was 70 kb long, and this element, designated Tn4653, completely included Tn4651. Genetic analysis of Tn4653 demonstrated that its transposition involves two steps, i.e. cointegrate formation and its subsequent resolution. The former step required a trans-acting factor, transposase, which was encoded in a 3.0 kb fragment at one end of Tn4653, and the latter step was inferred to be mediated by the factors necessary for resolution of the Tn4651-mediated cointegrate. The transposase functions were not interchangeable between the two transposons.

Analysis of the variable endpoints generated by one-ended transposition of Tn21

One-ended transposition of Tn21 generates recombinants usually containing a whole copy of the donor replicon plus a short duplication of it (S. Mötsch, R. Schmitt, P. Avila, F. de la Crue, E. Ward, and J. Grinsted, Nucleic Acids Res. 13:3335-3342, 1985). This work shows that recombinants containing less than a whole copy of the donor replicon (hereafter called short recombinants) could also be detected when plasmid donors which contained two selectable genetic markers were used. Short recombinants were produced at the same frequency from TnpR+ donor molecules as from TnpR- donor molecules in a RecA- background. Therefore, they were not resolution products of larger recombinants. This result invalidates a previous hypothesis to explain one-ended transposition, that is, that one-ended transposition arises from the use of secondary ends by the transposition apparatus. On the other hand, it suggests that one-ended transposition of Tn21 occurs via a simple insertion mechanism.