An integrative plasmid and multiple-sized plasmids of Pseudomonas syringae pv. phaseolicola have extensive homology

Genomic insights into the contribution of phytopathogenic bacterial plasmids to the evolutionary history of their hosts

Plasmids are common residents of phytopathogenic bacteria and contribute significantly to host evolution in a multi-faceted manner. Plasmids tend to encode determinants of virulence and ecological fitness that can enhance adaptation to a specific niche or can influence niche expansion. Many of these determinants appear to have been acquired from other bacteria via horizontal transfer, illustrating an important function of plasmids in the acquisition of sequences that enable rapid evolution. These genes can ultimately be delivered to the host chromosome through plasmid integration events, thus stabilizing important acquired determinants within the genome. Most plasmids characterized in phytopathogenic bacteria are self-transmissible and possess suites of genes encoding type IV secretion systems. In addition, the phytopathogenic bacterial plasmid "mobilome" includes insertion sequence and other transposable elements that contribute to the movement of sequences within and between genomes. Possession of mosaic and ever-changing plasmids allows phytopathogenic bacteria to maintain a dynamic, flexible genome and possible advantage in host-pathogen and other environmental interactions that belies the concept of plasmids as apparently selfish genetic elements.

Identification and mapping of regions that confer plasmid functions and of sites for excisive recombination of plasmid pMMC7105

Strain PP808 of Pseudomonas syringae pv. phaseolicola contains pEXC8080 (34.6 kb), the smallest of several plasmids that originated by partial excision of the cryptic plasmid, pMMC7105 (150 kb), from the host chromosome. This excision plasmid is derived entirely of sequences from pMMC7105 and contains a 24 kb region referred to as common DNA, which is present in each of the other excision plasmids. A six enzyme restriction endonuclease map was constructed of pEXC8080. The replication region was mapped by identifying small restriction fragments that conferred replication properties to pMB1 plasmids that otherwise fail to replicate in Pseudomonas. This region is located within the common DNA and is 0.8-3.8 kb in size. Sequences from pEXC8080 failed to stabilize pMB1 derivatives in Pseudomonas in the absence of antibiotic selection, but stability functions were mapped to a region of pMMC7105 that presumably remains integrated in the chromosome of strain PP808. An incompatibility region was mapped to a 7.3 kb region on pEXC8080 that is closely linked to, but not included within, the replication region. The recombination site was mapped to a 1.2 kb region of the fusion fragment that was formed upon excision of pEXC8080. RS-I, a repetitive sequence found on pMMC7105 was present in the fusion fragment at the site of recombination. RS-I was also mapped to BamHI fragments that recombined upon excision of pEXC8080 and suggest that it provides sites for homologous recombination.

Identification and cloning of genes involved in phaseolotoxin production by Pseudomonas syringae pv. "phaseolicola"

Genes involved in the production of phaseolotoxin by Pseudomonas syringae pv. "phaseolicola" NPS3121 were identified by Tn5 mutagenesis and cosmid cloning. A total of 5,180 kanamycin-resistant colonies were screened for the loss of phaseolotoxin production by a microbiological assay. Six independent, prototrophic, Tox- mutants were isolated that had Tn5 insertions in five different EcoRI fragments. All six mutants had Tn5 inserted in the same KpnI fragment, which had a length of ca. 28 kilobases including Tn5. The mutants produced residual toxin in vitro. An EcoRI fragment containing Tn5 and flanking sequences from mutant NPS4336 was cloned and used to probe a wild-type genomic library by colony hybridization. Seven recombinant plasmids showing homology to this probe were identified. Each Tox- mutant was restored in OCTase-specific toxin production by two or more of the recombinant plasmids. The data suggest that at least some of the genes involved in phaseolotoxin production were clustered in a large KpnI fragment. No homology was detected between the Tn5 target fragment cloned from mutant NPS4336 and the total genomic DNA from closely or distantly related bacteria that do not produce phaseolotoxin.

Indigenous plasmids in Pseudomonas syringae pv. tomato: conjugative transfer and role in copper resistance

Twenty strains of Pseudomonas syringae pv. tomato were examined for the presence of plasmid DNA. P. syringae pv. tomato plasmids were grouped into five size classes: class A ranged from 95 to 103 kilobases (kb); class B ranged from 71 to 83 kb; class C ranged from 59 to 67 kb; class D ranged from 37 to 39 kb; and class E was 29 kb. All strains contained at least two plasmids in classes A and B. The conjugative ability of P. syringae pv. tomato plasmids in three strains was demonstrated by mobilization of the nonconjugative plasmid RSF1010 into Pseudomonas syringae pv. syringae recipients. Plasmids from the three conjugative strains were labeled with Tn5. Four conjugative plasmids were identified by their repeated transfer to P. syringae pv. syringae recipients. P. syringae pv. tomato strains varied in sensitivity to copper sulfate (CuSO4): MICs were 0.4 to 0.6 mM for sensitive strains, 1.2 mM for moderately resistant strains, and 1.6 to 2.0 mM for very resistant strains. One very resistant strain, PT23, functioned as a donor of copper resistance. Recipient P. syringae pv. syringae strains PS51 and PS61 were inhibited by 0.1 mM CuSO4, whereas the CuSO4 MICs for transconjugant strains PS51(pPT23A) and PS61(pPT23C) were 1.8 and 2.6 mM, respectively. P. syringae pv. tomato strains PT12.2 and PT17.2 were inhibited by 0.6 mM copper sulfate, but their copper sulfate MICs were 2.6 and 1.8 mM, respectively, when they acquired pPT23C. Therefore, copper resistance in PT23 was controlled by two conjugative plasmids, designated pPT23A (101 kb) and pPT23C (67 kb).

Characterization of eight excision plasmids of Pseudomonas syringae pv. phaseolicola

Pseudomonas syringae pv. phaseolicola strain LR719 contains a 150 kilobase pair (kb) plasmid pMC7105, stably integrated into its chromosome. Occasionally, single colony isolates of this strain contain an excision plasmid. Eight unique excision plasmids were selected and characterized by BamHI restriction endonuclease and blot hybridization analyses. These plasmids ranged in size from 35 to 270 kb; the largest contained approximately 130 kb of chromosomal DNA sequences. Restriction maps of pMC7105 were developed to deduce the site of integration and to identify the fragments in which recombination occurred to produce each excision plasmid. The eight excision plasmids were arranged into five classes based on the sites where excision occurs. A 20 kb region of pMC7105, which includes BamHI fragment 9 and portions of adjacent fragments, is present in all excision plasmids and thought to contain the origin of replication. The site of integration on pMC7105 maps within BamHI fragment 8. This fragment shows homology with seven other BamHI fragments of pMC7105 and with five chromosomal fragments identified among the excision plasmids. The data strongly suggest that the integration of pMC7105 may have occurred at a repetitive sequence present on the chromosome and on the plasmid.