Analysis of IS21-mediated mobilization of plasmid pACYC184 by R68.45 in Escherichia coli

CpxAR two-component system contributes to virulence properties of Cronobacter sakazakii

Cronobacter sakazakii (C. sakazakii) is a foodborne pathogen which threaten susceptible hosts including infants. CpxA/CpxR, a regulatory two-component system (TSC), contributes to stress response and virulence in various Gram-negative pathogens, but its role in C. sakazakii has not been thoroughly studied. In this study, we constructed CpxA, CpxR, CpxAR deletion and complementation strains. The mutants showed weakened bacterial adhesion to and invasion of HBMEC and Caco-2, reduced intracellular survival and replication of C. sakazakii within RAW264.7 macrophages, and decreased translocation of HBMEC and Caco-2 monolayers. Mutants demonstrated lower levels of tight junction proteins disruption and reduced apoptosis and cytotoxicity in Caco-2 monolayer compared to wild type strain. CpxAR TCS deletion mutants demonstrate attenuated virulence in newborn mice, which was evidenced by fewer bacterial cells loads in tissues and organs, lower levels of intestinal epithelial barrier dysfuction and milder damages in intestinal tissues. All these phenotypes were recovered in complemented strains. In addition, qRT-PCR results showed that CpxAR TCS of C. sakazakii played roles in regulating the expression of several genes associated with bacterial virulence and cellular invasion. These findings indicate that CpxAR TCS is an important regulatory mechanism for virulence of C. sakazakii, which enrich our understanding of genetic determinants of pathogenicity of the pathogen.

Four different integrative recombination events involved in the mobilization of the gonococcal 5.2 kb beta-lactamase plasmid pSJ5.2 in Escherichia coli

We identified and characterized four different recombination mechanisms involved in the cointegrative transfer of the Neisseria gonorrhoeae beta-lactamase plasmid pSJ5.2 by the gonococcal 41 kb tet(M) and the Gram negative self-transmissible plasmids N3 and R64 drd-33 using an Escherichia colirecA-background. Mobilization of pSJ5.2 by the tet(M) plasmid occurred by cointegration through a replicative transposition of two IS1 elements inserted upstream from the beta-lactamase gene of pSJ5.2 and creating a IS1::beta-lactamase hybrid promoter. Two types of recombinational events occurred within the 1.8 kb BamH1-HindIII fragment of pSJ5.2 with the N3 and R64 plasmids. A non-homologous recombination was found at coordinates 1817 and 2849 of pSJ5.2 with sequences from R64. A non-homologous recombination combined with an IS26-mediated one-ended transposition was found at coordinates 1817 and 3010 of pSJ5.2 with N3. In both recombinational events, a deletion of over 1 kb of pSJ5.2 occurred. The fourth recombination event was detected in the 1.0 kb BamH1-HindIII fragment of pSJ5.2 by homologous recombination between DNA from the truncated Tn3 resolvase gene of pSJ5.2 and the resolvase sequences from R64 and N3.

Cointegrase, a naturally occurring, truncated form of IS21 transposase, catalyzes replicon fusion rather than simple insertion of IS21

The bacterial insertion sequence IS21 contains two genes, istA and istB, which are organized as an operon. IS21 spontaneously forms tandem repeats designated (IS21)2. Plasmids carrying (IS21)2 react efficiently with other replicons, producing cointegrates via a cut-and-paste mechanism. Here we show that transposition of a single IS21 element (simple insertion) and cointegrate formation involving (IS21)2 result from two distinct non-replicative pathways, which are essentially due to two differentiated IstA proteins, transposase and cointegrase. In Escherichia coli, transposase was characterized as the full-length, 46 kDa product of the istA gene, whereas the 45 kDa cointegrase was expressed, in-frame, from a natural internal translation start of istA. The istB gene, which could be experimentally disconnected from istA, provided a helper protein that strongly stimulated the transposase and cointegrase-driven reactions. Site-directed mutagenesis was used to express either cointegrase or transposase from the istA gene. Cointegrase promoted replicon fusion at high frequencies by acting on IS21 ends which were linked by 2, 3, or 4 bp junction sequences in (IS21)2. By contrast, cointegrase poorly catalyzed simple insertion of IS21 elements. Transposase had intermediate, uniform activity in both pathways. The ability of transposase to synapse two widely spaced IS21 ends may reside in the eight N-terminal amino acid residues which are absent from cointegrase. Given the 2 or 3 bp spacing in naturally occurring IS21 tandems and the specialization of cointegrase, the fulminant spread of IS21 via cointegration can now be understood.

Use of a xylE marker gene to monitor survival of recombinant Pseudomonas putida populations in lake water by culture on nonselective media

IncQ marker plasmids were previously constructed to enable the analysis of the survival of populations of Pseudomonas putida released into lake water (C. Winstanley, J. A. W. Morgan, R. W. Pickup, J. G. Jones, and J. R. Saunders, Appl. Environ. Microbiol. 55:771-777, 1989). We constructed equivalent IncP plasmids, pLV1016 and pLV1017, to provide conjugative alternative systems. Detection of the xylE gene carried by marker plasmids was found to be a valid indicator to use for studying the survival of released populations by culturing on nonselective media. These plasmids were used to study the survival of populations of Pseudomonas putida in both sterile and untreated lake water. The effects of inoculum size, the metabolic burden imposed on the cell by the unregulated expression of xylE, and an auxotrophic mutation carried by the host strain were studied. We also assessed the reproducibility and hence the predictability of the survival of released populations. Model systems with a single lake water sample and model systems with three different lake water samples, taken from the same site in consecutive months, were used to analyze variability between replicates and to assess differences caused by host strain or water sample. A large variability was found depending on which water sample was used. These findings imply that it will be difficult to predict accurately the survival of released populations in the natural environment.

Genetic structure, function and regulation of the transposable element IS21

The IncP plasmid R68.45 and other plasmids carrying tandem repeats of the insertion sequence IS21 [= (IS21)2] produce replicon fusions via transposition at high frequencies in Escherichia coli and other gram-negative bacteria, whereas plasmids with a single IS21 copy, e.g. R68, give replicon fusions rarely. The 2131 bp nucleotide sequence of IS21 was determined; at the ends there were 11 bp inverted repeats with one mismatch. Two adjacent open reading frames, istA and istB, were located on one DNA strand of IS21. In E. coli maxicells, polypeptides of 46 kDa (the istA gene product) and 30 kDa (the istB gene product) were expressed by (IS21)2 plasmids, but not by IS21 plasmids. Genetic analysis of (IS21)2 plasmids indicates that the IS21-IS21 junctions form a promoter, which initiates transcription of the istAB operon in one of the two IS21 elements. A single IS21 element fused to an inducible external tac promoter expressed both proteins after induction, but did not promote effective replicon fusion, unless an IS21-IS21 junction (the preferred site for IS21 transposase action) was also present on the plasmid carrying the tac-IS21 construct. The sequences located between the IS21 elements in (IS21)2, 3 bp in R68.45 or 2 bp in pME28, were not recovered in the replicon fusion products. Homologous recombination between the directly oriented IS21 elements in the fusion products led to plasmids with a single IS21 insertion. Analysis of the latter showed that IS21 had a low, but not totally random specificity of insertion and created target duplications of 4 bp (occasionally 5 bp).(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo generation of R68.45-pPGH1 hybrid plasmids conferring a Phl+ (meta pathway) phenotype

Plasmid pPGH1 originating from Pseudomonas putida strain H carries all the genes required for the degradation of phenol (or cresols) via the meta cleavage pathway. Besides mobilization of pPGH1 by a plasmid of the incompatibility group P-1, hybrid plasmids conferring the Phl+ phenotype could be selected, when R68.45 was the conjugative plasmid. The hybrids contain the complete R68.45 and part of pPGH1. Integration of Phl-DNA of pPGH1 into R68.45 occurred exclusively via the IS21 region of R68.45.

Behavior of the IncW plasmid Sa in Zymomonas mobilis

The stability of the broad-host-range IncW R plasmid Sa in Zymomonas mobilis ZM6100(Sa) was monitored using three antibiotic resistance markers carried by Sa. When grown in batch culture without selection, ZM6100(Sa) rapidly lost the Sa plasmid. When grown with selection for either kanamycin or spectinomycin resistance, the three Sa markers were retained in at least 90% of the population, with spontaneous loss of chloramphenicol resistance being observed in the rest of the population. When ZM6100(Sa) was grown with selection for chloramphenicol resistance, the Sa plasmid either formed a cointegrate plasmid, designated pNSW301, with a native ZM6100 plasmid or was lost except for the chloramphenicol resistance marker. Possible transportation of the chloramphenicol resistance region of Sa to the Z. mobilis chromosome was indicated by hybridization data. The cointegrate plasmid, pNSW301, was maintained stably, except for spontaneous loss of chloramphenicol resistance, in Z. mobilis without selection for 300 generations. pNSW301 was able to replicate in Escherichia coli and was capable of conjugal transfer.

Novel rearrangements of IS30 carrying plasmids leading to the reactivation of gene expression

Unusual DNA rearrangements involving the prokaryote mobile genetic element IS30 have been identified. In order to study the potential mechanisms for the reactivation of a gene after IS element insertion, IS30 was introduced between the lacUV5 promoter and the galK gene of the multicopy plasmid pFR100. In this plasmid terminators of RNA transcription in the sequence of IS30 prevent expression of the galK gene from the lacUV5 promoter. A number of independently isolated mutant plasmids re-expressing the galK gene were studied and shown to be tandemly repeated dimers of the original plasmid. However, the two copies of IS30 were tandemly repeated at one of the original sites of insertion, while at the other site IS30 and three base pairs were missing. The repeated copies of IS30 were separated by two base pairs, the same as those originally flanking the elements. An apparently identical mechanism generated cointegrates between a derivative of plasmid pFD51 carrying IS30 upstream of a promoterless galK gene and a derivative of plasmid pACYC177 carrying IS30 inserted into the beta-lactamase gene. This arrangement brought the galK gene under the control of the beta-lactamase promoter of pACYC177. A mechanism involving aborted conservative transposition of IS30 is discussed as a possible route for the generation of these novel cointegrates. In a third experiment we isolated an insert of IS30 which was also two base pairs away from an already resident IS30 element. This insertion of IS30 created a strong promoter of RNA transcription, which has the potential to increase the expression of the transposase in the downstream copy of the element.

Interactions between the transposable element IS21 on R68.45 and TN7 in Pseudomonas aeruginosa PAO

Tn7 transposes from the chromosome of Pseudomonas aeruginosa into the plasmid R68.45 with tandem IS21, at up to 400 times the frequency that it transposes into R68, which has only one copy of IS21. While R68::TN7 derivatives are stable, R68.45::Tn7 isolates undergo frequent deletions. Instability of R68.45 occurs whether Tn7 is inserted into the plasmid (cis configuration) or into the bacterial chromosome (trans configuration). The deletions of R68.45 start at the junction between the tandem IS21 copies and proceed clockwise, ending in the region of oriT. It appears that Tn7 and IS21 can mutually stimulate transposition of each other.

IS21 insertion in the trfA replication control gene of chromosomally integrated plasmid RP1: a property of stable Pseudomonas aeruginosa Hfr strains

Broad host range IncP-1 plasmids are able to integrate into the chromosome of gram-negative bacteria. Strains carrying an integrated plasmid can be obtained when the markers of a temperature-sensitive (ts) plasmid derivative are selected at non-permissive temperature; in this way Hfr (high frequency) donor strains can be formed. The integrated plasmids, however, tend to be unstable in the absence of continuous selective pressure. In order to obtain stable Hfr donor strains of Pseudomonas aeruginosa PAO, we constructed a derivative of an RP1 (ts) plasmid, pME134, which was defective in the resolvase gene (tnpR) of transposon Tn801. Chromosomal integration of pME134 was selected in a recombination-deficient (rec-102) PAO strain at 43 degrees C. Plasmid integration occurred at different sites resulting in a useful set of Hfr strains that transferred chromosomal markers unidirectionally. The tnpR and rec-102 mutations prevented plasmid excision from the chromosome. In several (but not all) Hfr strains that grew well and retained the integrated plasmid at temperatures below 43 degrees C, the insertion element IS21 of RP1 was found to be inserted into the trfA locus (specifying an essential trans-acting replication function) of the integrated plasmid. One such Hfr strain was rendered rec+; from its chromosome the pME134::IS21 plasmid (= pME14) was excised and transferred by conjugation to Escherichia coli where pME14 could replicate autonomously only when a helper plasmid provided the trfA+ function in trans. Thus, it appears that trfA inactivation favours the stability of chromosomally integrated RP1 in P. aeruginosa.

pULB113, an RP4::mini-Mu plasmid, mediates chromosomal mobilization and R-prime formation in Erwinia amylovora, Erwinia chrysanthemi, and subspecies of Erwinia carotovora

The RP4::mini-Mu plasmid pULB113, transferred from Escherichia coli strain MXR, was stable and transfer proficient in Erwinia amylovora strain EA303, E. carotovora subsp. atroseptica strain ECA12, E. carotovora subsp. carotovora strain ECC193, and E. chrysanthemi strain EC183. The plasmid mobilized an array of Erwinia sp. chromosomal markers (E. amylovora: his+,ilv+,rbs+,ser+,thr+;E. chrysanthemi:arg+,his+,ilv+,leu+; E. carotovora subsp. atroseptica: arg+,gua+,leu+,lys+,pur+,trp+; E. carotovora subsp. carotovora: arg+,gua+,leu+,lys+,out+[export of enzymes],pur+,trp+), suggesting random interactions of the plasmid with the chromosomes. In E. carotovora subsp. carotovora, pULB113-mediated two-factor crosses revealed linkage between three auxotrophic markers and the out loci. The export of pectate lyase, polygalacturonase, and cellulase and the maceration of potato tuber tissue occurred with Out+, but not Out-, strains of E. carotovora subsp. carotovora, indicating the importance of enzyme export in plant tissue maceration. Erwinia sp. donors harboring pULB113 complemented mutations in various biosynthetic and catabolic genes (arg, gal, his, leu, met, pro, pur, thy) in Escherichia coli recA strains. Escherichia coli transconjugants harbored pULB113 primes as indicated by the cotransfer of Erwinia genes and pULB113 markers and a change in plasmid mass. Moreover, the PstI and SmaI cleavage patterns of selected pULB113 primes were different from those of pULB113. pULB113 primes carried DNA insertions ranging from 3 to about 160 kilobases. These findings indicate that pULB113 is useful for in vivo gene cloning and genetic analysis of various enterobacterial phytopathogens.

Vector insertion mutagenesis of Rhizobium sp. strain ORS571: direct cloning of mutagenized DNA sequences

When the limited-host-range plasmid pVP2021 carrying Tn5 was mobilized into Rhizobium sp. strain ORS571 and stable acquisition of Tn5 was selected, ORS571 plasmid-genome cointegrates were exclusively obtained; direct Tn5 transposition was never observed. In every case, genomic cointegrates exhibited an additional (third) IS50 element that bordered VP2021 DNA sequences but maintained a single Tn5 element. Genomic cointegrates containing IS50 triplications were stable; neither phenotypic reversion nor resolution was detectable. Auxotrophic mutant strains (vector insertion mutants) were identified at expected frequencies among derivatives carrying ostensibly random genomic pVP2021 insertions; N2 fixation (Nif)-defective vector insertion mutants were observed among these derivatives at a frequency of 10(-3). The presence of integrated pVP2021 in ORS571 nif::VP2021 mutant genomes enabled VP2021 to constitute an endogenous cloning vector. After EcoRI or KpnI digestions, genomic nif::pVP2021 DNA sequences contiguous with integrated pVP2021 were directly cloned as new replicons without addition of an exogenous vector. Recombinant plasmids derived from two such nif::pVP2021 mutants hybridized to previously analyzed ORS571 Nif DNA sequences. Recombinant plasmid DNA and ORS571 Nif region DNA were found to be colinear; pVP2021 insertions could be accurately mapped. pVP2021 insertion-mutagenesis thus allows the direct cloning of ORS571 gene sequences for which mutant phenotypes can be selected or screened.