Structural instability of IncP-1 plasmids in Pseudomonas aeruginosa PAT involves interaction with plasmid pVS1

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.

Specific-purpose broad-host-range vectors

Several plasmid derivatives of broad-host-range Inc P4 plasmid RSF1010 were constructed and characterized. Vector pAYC30 was constructed by insertion in vivo into the genome of RSF1010 the Hgr transposon Tn501, originating from the plasmid pVS1 of Pseudomonas aeruginosa. Plasmids with inserts of PstI or SacI fragments may be selected by inactivation of genes sul and aph, respectively. The cloning at unique site SalGI leads to the appearance of HgCl2--sensitive transformants. Versatile cloning vector pAYC1 consists of two replicons, RSF1010 and plasmid pMZ7, a derivative of R6K. The constructed plasmid is 16.9 kb in length and determines resistance to five drugs. Two promoter-probe broad-host-range vectors, pAYC36 and pAYC37, were obtained by replacing a small segment from the DNA sequence of the aph gene promoter of previously described plasmid pAYC32 with the polylinker from plasmid pUC19. Therefore, vector plasmids retained the intact gene aph (Smr); however, they have Sms phenotype because of the insertional inactivation of the promoter. The genetic structure of promoter-probe vectors allows one to select clones, containing hybrid plasmids with an active promoter for gene aph expression.

Cloning vectors derived from the Pseudomonas plasmid pVS1

The Pseudomonas plasmid pVS1, which has about seven copies, was reduced to a minimal replicon and used to construct stable gene-cloning vehicles. The host for all cloning experiments was P. aeruginosa strain PAO. Two nonmobilizable plasmids, pME260 and pME290, and one RP1-mobilizable plasmid, pME285, were constructed. The vectors pME260 (6.3 kb) and pME290 (6.8 kb) carry the Tn801 bla gene specifying carbenicillin (Cb) resistance, a good selective marker in Pseudomonas, and the Tn903 aph gene encoding kanamycin (Km) resistance, with useful restriction sites for insertional inactivation. The Mob+ vector pME285 (10.6 kb) carries the aph gene and the Tn501-derived merRTCA genes coding for mercuric ion resistance, another good selective marker in Pseudomonas. The hypothetical merD gene, which may follow the merA gene in Tn501 but is absent from pME285, appeared to be dispensable for mercuric ion resistance in P. aeruginosa. The Mob- vector pME290 could be introduced by transformation and maintained in strains of P. aeruginosa, P. fluorescens, P. putida, P. acidovorans, P. stutzeri, P. mendocina, P. cepacia, and P. syringae. The plasmid was compatible with IncP-1 and IncP-4 replicons.

Genetic and molecular characterization of the Pseudomonas plasmid pVS1

A restriction map of the 30-kb nonconjugative Pseudomonas plasmid pVS1 was constructed. Derivatives of pVS1 obtained in vitro by successive deletions were used to localize on the physical map the determinant for resistance to mercuric ions (carried by transposon Tn501), the gene(s) encoding sulfonamide resistance, a 1.6-kb region affecting plasmid stability and establishment in P. fluorescens ATCC 13525, and a segment required for mobilization of pVS1 by plasmid RP1. The sulfonamide resistance determinant of pVS1 appeared to be closely related to that of transposon Tn21. A mini-pVS1 replicon, pME259, consisting of an essential 1.55-kb segment (designated rep and thought to carry the origin of replication) and a mercury resistance determinant was able to replicate P. aeruginosa PAO but selective pressure was needed for plasmid maintenance. The copy number of pVS1 derivatives was estimated to be 6-8 per chromosome equivalent. Plasmids possessing the essential rep segment plus the adjacent stability region could be established in strains of P. aeruginosa, P. putida, P. fluorescens, P. acidovorans, P. cepacia, P. mendocina, P. stutzeri, P. syringae, Agrobacterium tumefaciens, and Rhizobium leguminosarum.

Construction of recombination-deficient strains of Pseudomonas aeruginosa

The rec-102 mutation had pleiotropic effects in Pseudomonas aeruginosa: low recombinational proficiency in conjugation and transduction; high UV sensitivity; inability to induce pyocin R2 by mitomycin C; and increased susceptibility to mitomycin C and nalidixic acid. The rec-102 locus was mapped by R68.45-mediated conjugation in the 45 min region of the PAO chromosome, between argF and thr-9001. By selection for a marker in this region, rec-102 can be introduced into a P. aeruginosa strain of interest using an R68.45 rec-102 donor. The recombination-deficient strains constructed in this way were phenotypically similar to Escherichia coli recA mutants.

Spontaneous deletions of the chromosome-mobilizing plasmid R68.45 in Pseudomonas aeruginosa PAO

In Pseudomonas aeruginosa strain PAO the chromosome-mobilizing IncP-1 plasmid R68.45 was unstable whereas the parent plasmid R68 was stable. The instability of R68.45 was observed in rec+ and rec strains within about 100 generations after conjugal transfer of the plasmid and, to a lesser extent, in established R68.45 donor strains. Two phenotypically distinct classes of R68.45 derivatives were obtained: (i) CbR (carbenicillin-resistant), TcR (tetracycline-resistant), KmR (kanamycin-resistant), Tra+ (transfer proficient), Cma- (chromosome-mobilizing ability), lacking the duplicated IS21 copy typical of R68.45 and indistinguishable from R68 by restriction enzyme analysis; (ii) CbR TcR KmS Tra- Cma-, due to deletion of one IS21 copy, the adjacent KmR gene, and a variable part of the Tra-1 region including, in most cases, the origin of transfer (oriT). Both types of deletion derivatives were stable. R68.45 derivatives lacking the Tra-2 region were not recovered spontaneously, but could be constructed in vitro and were stable in strain PAO. Deletion formation of type ii as well as Cma did not depend on homologous recombination and can be ascribed to functions of the duplicated IS21. Chromosome mobilization does not appear to require obligatory transfer of the entire R68.45 plasmid. Four ClaI restriction sites were mapped on R68 extracted from P. aeruginosa. One of these sites was cryptic, presumably because of methylation, when the plasmid was prepared from Escherichia coli (dam+).