A molecular analysis of transductional marker rescue involving P-group plasmids in Pseudomonas aeruginosa

Tn6603, a Carrier of Tn5053 Family Transposons, Occurs in the Chromosome and in a Genomic Island of Pseudomonas aeruginosa Clinical Strains

Transposons of the Pseudomonasaeruginosa accessory gene pool contribute to phenotype and to genome plasticity. We studied local P. aeruginosa strains to ascertain the encroachment of mer-type res site hunter transposons into clinical settings and their associations with other functional modules. Five different Tn5053 family transposons were detected, all chromosomal. Some were solitary elements; one was in res of Tn1013^#, a relative of a reported carrier of int-type res site hunters (class 1 integrons), but most were in res of Tn6603, a new Tn501-related transposon of unknown phenotype. Most of the Tn6603::Tn elements, and some Tn6603 and Tn6603::Tn elements found in GenBank sequences, were at identical sites in an hypothetical gene of P. aeruginosa genomic island PAGI-5v. The island in clonally differing strains was at either of two tRNA^Lys loci, suggesting lateral transfer to these sites. This observation is consistent with the membership of the prototype PAGI-5 island to the ICE family of mobile genetic elements. Additionally, the res site hunters in the nested transposons occupied different positions in the Tn6603 carrier. This suggested independent insertion events on five occasions at least. Tn5053 family members that were mer-/tni-defective were found in Tn6603- and Tn501-like carriers in GenBank sequences of non-clinical Pseudomonas spp. The transposition events in these cases presumably utilized tni functions in trans, as can occur with class 1 integrons. We suggest that in the clinical context, P. aeruginosa strains that carry Tn6603 alone or in PAGI-5v can serve to disseminate functional res site hunters; these in turn can provide the requisite trans-acting tni functions to assist in the dissemination of class 1 integrons, and hence of their associated antibiotic resistance determinants.

DDS promotes longevity through a microbiome-mediated starvation signal

The antibiotic diaminodiphenyl sulfone (DDS) is used in combination with other antibiotics as a first line treatment for leprosy. DDS has been previously reported to extend lifespan in Caenorhabditis elegans through inhibition of pyruvate kinase and decreased mitochondrial function. Here we report an alternative mechanism of action by which DDS promotes longevity in C. elegans by reducing folate production by the microbiome. This results in altered methionine cycle metabolite levels mimicking the effects of metformin and lifespan extension that is dependent on the starvation- and hypoxia-induced flavin containing monoxygenase, FMO-2.

Excessive folate synthesis limits lifespan in the C. elegans: E. coli aging model

Background

Gut microbes influence animal health and thus, are potential targets for interventions that slow aging. Live E. coli provides the nematode worm Caenorhabditis elegans with vital micronutrients, such as folates that cannot be synthesized by animals. However, the microbe also limits C. elegans lifespan. Understanding these interactions may shed light on how intestinal microbes influence mammalian aging.

Results

Serendipitously, we isolated an E. coli mutant that slows C. elegans aging. We identified the disrupted gene to be aroD, which is required to synthesize aromatic compounds in the microbe. Adding back aromatic compounds to the media revealed that the increased C. elegans lifespan was caused by decreased availability of para-aminobenzoic acid, a precursor to folate. Consistent with this result, inhibition of folate synthesis by sulfamethoxazole, a sulfonamide, led to a dose-dependent increase in C. elegans lifespan. As expected, these treatments caused a decrease in bacterial and worm folate levels, as measured by mass spectrometry of intact folates. The folate cycle is essential for cellular biosynthesis. However, bacterial proliferation and C. elegans growth and reproduction were unaffected under the conditions that increased lifespan.

Conclusions

In this animal:microbe system, folates are in excess of that required for biosynthesis. This study suggests that microbial folate synthesis is a pharmacologically accessible target to slow animal aging without detrimental effects.

Posttranscriptional regulation of the inducible nonenzymatic chloramphenicol resistance determinant of IncP plasmid R26

The inducible nonenzymatic chloramphenicol resistance (Cmr) determinant of the IncP plasmid R26 was cloned on a 1,900-base-pair restriction endonuclease HindIII fragment. Transposon Tn5 mutagenesis revealed that at least 1,400 base pairs is required for expression of Cmr. There was no increase in the level of Cmr when the copy number of the determinant was raised by cloning in pBR322 or pUB5572. Expression of Cmr by cells carrying a lower-copy-number pUB5572cml+ plasmid was inducible and thus indistinguishable from those with R26 itself. However, pBR322cml+-carrying cells expressed Cmr constitutively, possibly due to the activity of vector promoters or an elevated copy number. Transcriptional and translational cml-lac fusions were constructed. The operon (transcriptional) cml-lac fusion carried by the low-copy-number plasmid pUB5572 caused a low level of constitutive beta-galactosidase activity, which could not be elevated by induction with chloramphenicol and was not affected by a coresident R26cml+ element. In contrast, the gene (translational) cml-lac fusion expressed low-level beta-galactosidase activity, which was elevated fivefold by prior exposure to chloramphenicol. We conclude that the regulation of Cmr occurs posttranscriptionally.

Location of a function on RP1 that fertility inhibits Inc W plasmids

Two fertility inhibition (Fi+) functions which reduce R388 (Inc W) transfer were detected on RP1 (Inc P). Neither function affected R388 -mediated surface exclusion but they could be distinguished by their effect on pilus production. One of the functions was located in the 6.5-kb Pst1 -C region of RP1, part or all of which also occurs on six Fi+ but not two Fi- Inc P plasmids studied.

Nonenzymatic chloramphenicol resistance determinants specified by plasmids R26 and R55-1 in Escherichia coli K-12 do not confer high-level resistance to fluorinated analogs

Plasmids R26 (Inc P) and R55-1 (Inc C) specify inducible nonenzymatic resistance to chloramphenicol. Escherichia coli K-12 strains harboring these plasmids encoded low-level resistance to thiamphenicol analogs Sch 25298 and Sch 25393 but failed to specify resistance to the fluorinated chloramphenicol analog Sch 24893. The analogs were efficient inducers of high-level chloramphenicol resistance.

A chromosomally located transposon in Pseudomonas aeruginosa

A new transposon, Tn2521, coding for carbenicillin, streptomycin, spectinomycin, and sulfanilamide resistance, has been identified in Pseudomonas aeruginosa. The transposon occurs naturally in the chromosome of clinical strains of P. aeruginosa isolated in geographically separated hospitals. This has been demonstrated by its transductional linkage to the pur-136 marker and also by Southern hybridization. Tn2521 is 6.8 kilobases, can transpose from the chromosome to both IncP-1 and IncP-2 plasmid genomes, and has a pattern of restriction endonuclease sites unlike that of any previously described transposon. The carbenicillin resistance carried by Tn2521 is due to the PSE-4 type of beta-lactamase.

Plasmid-encoded lysine decarboxylation in Proteus morganii

As a rule, Proteus morganii does not decarboxylate lysine. However, lysine-positive P. morganii strains have been recently described. We suspected a plasmid origin for this atypical character, and we analyzed 14 strains to study this question. Among these strains, 8 yielded lysine-negative segregants after acridine orange or ethidium bromide treatment, and 10 transferred their lysine-positive character to a recipient P. morganii strain. All of the 14 strains analyzed at least segregated or conjugated. Three lysine-positive transconjugants, in turn, segregated lysine-negative variants after ethidium bromide treatment. The eight wild-type lysine-positive strains that segregated lysine-negative subclones contained a large (35 to 45 megadaltons) plasmid detectable by agarose gel electrophoresis. Similarly, the 10 lysine-positive transconjugants contained a plasmid of the same size, whereas P. morganii 1000, the recipient strain, did not contain any detectable plasmid. The large plasmid clearly disappeared in 9 of 11 lysine-negative segregants analyzed. It is concluded that the lysine-positive character of these P. morganii strains is plasmid encoded, and the taxonomical implications are discussed.

Transposable plasmid deoxyribonucleic acid sequence in Pseudomonas aeruginosa which mediates resistance to gentamicin and four other antimicrobial agents

A 9.1 x 10(6)-dalton transposable deoxyribonucleic acid sequence resides within Pseudomonas aeruginosa plasmid R1033 and mediates resistance to gentamicin, streptomycin, sulfamethoxazole, chloramphenicol, and mercuric chloride. Transposability was demonstrated in Escherichia coli when this sequence, designated Tn1696, excised from R1033 and integrated into plasmid pMB8. Excision and insertion of Tn1696 occurred independently of the host Rec phenotype and may involve the 140-base pair, inverted deoxyribonucleic acid repeated region that flanks this sequence. Occurrence of a multiresistance transposon on a transferrable plasmid that has a broad host range may have serious epidemiological and therapeutic consequences.

Tn951: a new transposon carrying a lactose operon

A new transposon, Tn951, is described, which derives from plasmid pGC1, originally isolated from Yersinia enterocolitica. Tn951 is 16.6 kb long and presumably flanked by small inverted repeats. It carries the lac genes i, z and y. This lac system is homologous to the E. coli lac operon. However, homology is restricted to 5.6 kb. The DNA sequences surrounding the lac operons on Tn951 and E. coli are nonhomologous. This leads to speculations about the origin of the E. coli lac operon itself.

Evidence for transductional shortening of the plasmid obtained by recombination between the TOL catabolic plasmid and the R91 R plasmid

The previously isolated plasmid pND3, arising from recombination between the TOL catabolic plasmid and the R plasmid R91, was transduced by pf16 inPseudomonas putida. Apparent transductional shortening was evident in 25% of the transduced pND3 plasmids. Transductants were isolated which had segregated the antibiotic resistance marker, transfer ability and some of the catabolic functions of the parent plasmid.

Isolation and characterisation of deletion mutants involving the transfer genes of P-group plasmids in Pseudomonas aeruginosa

The P-group plasmids RP1 and R26 are recovered at low frequency following conjugal transfer to B3-lysogens of P. aeruginosa PAO. The rare carbenicillin-resistant transcipients that do arise are usually transfer-defective (Tra-) and may show the loss of other plasmid borne functions, namely kanamycin-resistance (Kmr) and reduced plating of phage G101 (Spp+). The four phenotypic classes that occur among the Tra- derivatives are respectively, Tra- (69-81%), Tra- Spp- (12-30%), Tra- Kms and Tra- Kms Spp- (0.2-1%), of which the latter three are dut to plasmid deletions. This is seen from the sizes of the plasmids carried by these bacteria and from the transductional analysis of the R26-derivatives. Thus, although R26 (MW = 52 X 106 daltons) is too large to be transduced by phage F116L (MW = 40 X 106), this is possible for its Tra- Kms and Tra Kms Spp- derivatives. The phenotypes and frequencies of the various transcipient classes suggests that the gene order Km.. Tra.. Spp occurs in both RP1 and R26, and that Spp is more closely linked to Tra than is Km. These conclusions are supported by the sizes of the plasmid mutants since deletions spanning the loci Km Tra Spp, Km Tra, and Tra Spp involve the loss of DNA of MW 8-17 X 106, 5-13 X 106 AND 1-9 X 106 DALTONS RESPECTIVELY. Whilst all the transcipients displayed the incompatibility properties of the parent plasmids (Inc+), only some retanied plasmid surface exclusion (Sfx+). Moreover, a strict correlation existed between the Sfx and Spp phenotypes such that the transcipients were either wild type, Sfx- Spp-, or displayed an intermediate phenotype for both characters. Thir are different manifestations of the same gene function. The deletion map of these various markers in both RP1 and R26 therefore seems to be Km.. Tra.. Sfx/Spp.. Inc.

The properties of hybrids formed between the P-group plasmid RP1 and various plasmids from Pseudomonas aeruginosa

R38, R931-1, and R933 are conjugative plasmids derived from strains of Pseudomonas aeruginosa. They confer resistance to mercuric ions (Hg-r), and do not tranfer from P. aeruginosa to Escherichia coli at detectable frequencies. Hybrids between each of these plasmids and the P-group plasmid, RP1, have been detected among the rare Hg-r transconjugants arising from matings of P. aeruginosa PAO donors (RP1 + R+) and E. coli K12 recipients. Two independently isolated hybrid plasmids from each of the three mating combinations have been studied. All were found to confer the entire marker phenotype of RP1, but only the Hg-r phenotype of their second parent. Moreover, all were larger than RP1 but comprised only two groups of sizes; those increased by about 14 x 10(6) daltons (the RP1/R38 hybrids), and those increased by about 30 x 10(6) daltons (the RP1/R931-1 and RP1/R933 hybrids). The hybrid plasmids were all too large to be transduced intact by phage F116L, but tranduction of fragments was possible. Thus, the determinants for both carbenicillin-resistance (Cb-r) (from RP1) and mercuric-ion-resistance could be "rescued" by recipients that already carried an RP1-like plasmid and were recombination-proficient. A molecular analysis of the plasmids recovered from such transductants suggested that each of the parental hybrids was comprised of an entire RP1 genome into which a fragment of heterologous DNA had been inserted. In similar experiments in which the recipient carried a derivative of R931-1, the Hg-r but not the Cb-r determinant could be rescued. This suggested that R38, R931-1, and R933 shared sufficient homology in the region of the mer gene for recombination to occur between them. The reason for the inability to rescue the Cb-r determinant was also investigated.

Recombination between plasmids of incompatibility groups P-1 and P-2

R plasmids of incompatibility group P-2 are readily transmissible between Pseudomonas strains, but not to Escherichia coli or other enterobacteria, whereas those of group P-1 have a broad host range. Pseudomonas aeruginosa donor strains carrying both a P-1 plasmid (RP1, RP4, or R751) and a P-2 plasmid (pMG1, pMG2, pMG5, or RPL11) were mated with E. coli K-12, and selection was imposed for resistance markers on the P-2 plasmids. Transconjugants were obtained at a low frequency, in which P-2 markers were expressed and were serially transmissible in E. coli together with P-1 markers. These plasmids had P-1 incompatibility properties, conferred susceptibility to phages active on P-1 carrying strains, and behaved on sucrose gradient centrifugation as unimolecular species of higher molecular weights than the P-1 parent. Recombinant plasmid formation was independent of a functional Rec gene in both donor and recipient and, with R751, had a preferred site leading to loss of trimethoprim resistance. Interaction between insertion sequences may be involved. Thus, plasmids of group P-2 can recombine with R factors of another group quite separate in compatibility properties, host range, and pilus type. Formation of such recombinants provides one pathway by which the genetic diversity of plasmids may have evolved.