The multidrug resistance IncA/C transferable plasmid encodes a novel domain-swapped dimeric protein-disulfide isomerase

A comprehensive list of genes required for the efficient conjugation of plasmid Rts1 was determined by systematic deletion analysis

While conjugation-related genes have been identified in many plasmids by genome sequencing, functional analyses have not yet been performed in most cases, and a full set of conjugation genes has been identified for only a few plasmids. Rts1, a prototype IncT plasmid, is a conjugative plasmid that was originally isolated from Proteus vulgaris. Here, we conducted a systematic deletion analysis of Rts1 to fully understand its conjugation system. Through this analysis along with complementation assays, we identified 32 genes that are required for the efficient conjugation of Rts1 from Escherichia coli to E. coli. In addition, the functions of the 28 genes were determined or predicted; 21 were involved in mating-pair formation, three were involved in DNA transfer and replication, including a relaxase gene belonging to the MOBH12 family, one was involved in coupling, and three were involved in transcriptional regulation. Among the functionally well-analysed conjugation systems, most of the 28 genes showed the highest similarity to those of the SXT element, which is an integrative conjugative element of Vibrio cholerae. The Rts1 conjugation gene set included all 23 genes required for the SXT system. Two groups of plasmids with conjugation systems nearly identical or very similar to that of Rts1 were also identified.

Evolution and typing of IncC plasmids contributing to antibiotic resistance in Gram-negative bacteria

The large, broad host range IncC plasmids are important contributors to the spread of key antibiotic resistance genes and over 200 complete sequences of IncC plasmids have been reported. To track the spread of these plasmids accurate typing to identify the closest relatives is needed. However, typing can be complicated by the high variability in resistance gene content and various typing methods that rely on features of the conserved backbone have been developed. Plasmids can be broadly typed into two groups, type 1 and type 2, using four features that differentiate the otherwise closely related backbones. These types are found in many different countries in bacteria from humans and animals. However, hybrids of type 1 and type 2 are also occasionally seen, and two further types, each represented by a single plasmid, were distinguished. Generally, the antibiotic resistance genes are located within a small number of resistance islands, only one of which, ARI-B, is found in both type 1 and type 2. The introduction of each resistance island generates a new lineage and, though they are continuously evolving via the loss of resistance genes or introduction of new ones, the island positions serve as valuable lineage-specific markers. A current type 2 lineage of plasmids is derived from an early type 2 plasmid but the sequences of early type 1 plasmids include features not seen in more recent type 1 plasmids, indicating a shared ancestor rather than a direct lineal relationship. Some features, including ones essential for maintenance or for conjugation, have been examined experimentally.

Evolution in situ of ARI-A in pB2-1, a type 1 IncC plasmid recovered from Klebsiella pneumoniae, and stability of Tn4352B

The IncC plasmid pB2-1, from a Klebsiella pneumoniae isolate recovered in Brisbane prior to 1995, belongs to a subtype of type 1 IncC plasmids, here designated type 1a, that includes those carrying carbapenem resistance genes such as bla_NDM and bla_KPC. pB2-1 carries a 2358bp deletion in the rhs1 gene found in four other type 1a IncC plasmids. pB2-1 confers resistance to ampicillin, gentamicin, kanamycin, neomycin, tobramycin, sulfamethoxazole, tetracycline and trimethoprim. It transferred at a frequency of 4.7×10^-3 transconjugants per donor, similar to that of another type 1a plasmid pDGO100 but ten-fold lower than for its closest relative pRMH760. This difference may be due to a single amino acid substitution in TraL. pB2-1 has an ISEc52 insertion in the dsbC gene, demonstrating that dsbC is not essential for transfer. pB2-1 lacks the ARI-B insertion and hence the sul2 gene. The resistance genes sul1, dfrA10, aphA1a, bla_TEM, aadB, and tetA(B) are all in the ARI-A island, in a configuration that has evolved from ARI-A of pRMH760 in two steps. A 10.3kb segment extending from the catA1 gene to the end of pDUmer module was lost via homologous recombination between two copies of IS4321. In addition, a 5.3kb segment extending from IS1326 to the left end of Tn4352B was replaced with an 18.7kb tet(B)-containing segment bounded on one end by IS1 and on the other by IS26. The IS26-bounded transposon Tn4352B was shown to be stable in K. pneumoniae in contrast to the high instability observed in E. coli.

A plasmid-encoded DsbA homologue is a growth-phase regulated thioredoxin

The Pseudomonas aeruginosa plasmid pUM505 contains in a pathogenicity island the dsbA2 gene, which encodes a product with similarity to DsbA protein disulfide isomerases, enzymes that catalyze formation and isomerization of disulfide bonds in protein cysteine residues. Using transcriptional fusions, it was found that dsbA2 gene promoter is activated during the stationary phase, suggesting that DsbA2 protein may be required for adaptive changes that occur during this stage of bacterial growth. Transfer of the pUM505 dsbA2 gene to a cadmium-sensitive P. aeruginosa PAO1-derivative affected in the chromosomal dsbA gene, restored cadmium resistance, suggesting a role of DsbA2 in protecting protein disulfide bonds. PAO1 dsbA2 transformants displayed increased sensitivity to intercalating agent mitomycin C, indicating that DsbA2 functions as a thioredoxin enzyme able to modify and activate toxicity of this compound. These results highlight the adaptive role of the pUM505 plasmid in its P. aeruginosa hosts.

Persistence of Multi-Drug Resistance Plasmids in Sterile Water under Very Low Concentrations of Tetracycline

The persistence of the multi-drug resistance plasmids pAQU1 and IncFIB was examined in bacterial populations under very low selective pressure. We herein demonstrated that these plasmids stably remained not only in the original host, but also in a transconjugant, even after being in a non-culturable state. In seawater microcosms containing Photobacterium damselae 04Ya311 possessing pAQU1, no significant loss of pAQU1 was observed during a 30-d starvation period. The copy numbers of pAQU1 and IncFIB in E. coli were constant. The results of the present study suggest that these plasmids have the ability to remain among various bacteria under oligotrophic conditions with low antibiotic selection pressure.

Rv2466c mediates the activation of TP053 to kill replicating and non-replicating Mycobacterium tuberculosis

The emergence of multidrug- and extensively drug-resistant strains of Mycobacterium tuberculosis highlights the need to discover new antitubercular agents. Here we describe the synthesis and characterization of a new series of thienopyrimidine (TP) compounds that kill both replicating and non-replicating M. tuberculosis. The strategy to determine the mechanism of action of these TP derivatives was to generate resistant mutants to the most effective compound TP053 and to isolate the genetic mutation responsible for this phenotype. The only non-synonymous mutation found was a g83c transition in the Rv2466c gene, resulting in the replacement of tryptophan 28 by a serine. The Rv2466c overexpression increased the sensitivity of M. tuberculosis wild-type and resistant mutant strains to TP053, indicating that TP053 is a prodrug activated by Rv2466c. Biochemical studies performed with purified Rv2466c demonstrated that only the reduced form of Rv2466c can activate TP053. The 1.7 Å resolution crystal structure of the reduced form of Rv2466c, a protein whose expression is transcriptionally regulated during the oxidative stress response, revealed a unique homodimer in which a β-strand is swapped between the thioredoxin domains of each subunit. A pronounced groove harboring the unusual active-site motif CPWC might account for the uncommon reactivity profile of the protein. The mutation of Trp28Ser clearly predicts structural defects in the thioredoxin fold, including the destabilization of the dimerization core and the CPWC motif, likely impairing the activity of Rv2466c against TP053. Altogether our experimental data provide insights into the molecular mechanism underlying the anti-mycobacterial activity of TP-based compounds, paving the way for future drug development programmes.