Plasmids of Legionella

Two of the three sequenced L. pneumophila strains carry plasmids that might play a role either in adaptation to the environment or in virulence as both encode putative virulence factors, mobile genetic elements and antibiotic resistance genes. In strain Paris a 132 kb plasmid and in strain Lens a 60 kb plasmid was identified, both of which seem to be single copy plasmids. These plasmids contain a 24 kb

Legionella Genome

Figure 1.5. Regions of plasticity identified in the L. pneumophila genomes and their chromosomal locations in the three sequenced strains. Outer circle: Strain Paris; main specific regions are indicated in light grey; Central circle: Strain Lens, main specific regions are indicated in dark grey; Inner circle: Strain Philadelphia 1, main specific regions are indicated in grey. dot/icm, Dot/Icm type IV secretion system; lvh, Lvh type IV secretion system; efflux cluster; efflux pumps located on a plasticity zone of 130 kb (strain Lens), 135 kb (strain Philadelphia 1) and 87 kb (strain Lens).

Figure 1.5. Regions of plasticity identified in the L. pneumophila genomes and their chromosomal locations in the three sequenced strains. Outer circle: Strain Paris; main specific regions are indicated in light grey; Central circle: Strain Lens, main specific regions are indicated in dark grey; Inner circle: Strain Philadelphia 1, main specific regions are indicated in grey. dot/icm, Dot/Icm type IV secretion system; lvh, Lvh type IV secretion system; efflux cluster; efflux pumps located on a plasticity zone of 130 kb (strain Lens), 135 kb (strain Philadelphia 1) and 87 kb (strain Lens).

and 30 kb gene cluster, respectively, encoding homologs of Tra proteins (Figure. 1.6). Furthermore several mobile elements like transposases and phage-related proteins are present and about half of the genes code proteins of unknown function. Both plasmids also encode a paralog of CsrA—a protein known to act as repressor of transmission traits of L. pneumophila and as activator of replication (Molofsky and Swanson 2003), revealing the question whether these plasmids are implicated in virulence. The plasmid identified in strain Paris carries many genes coding proteins probably conferring antibiotic resistances, like a betalactamase, two spectin-omycin 3' adenylyltransferases or an aminoglycoside 3' phosphotransferase. Aminoglycoside phosphotransferase (3')-IIIa (APH) is a bacterial kinase that confers antibiotic resistance to many pathogenic bacteria and shares structural homology with eukaryotic protein kinases. The identified betalactamase is 93% similar to a previously identified D beta-lactamase determinant isolated from Legionella (Fluoribacter) gormanii ATCC 33297(T) where it was shown to transfer resistance to oxacillin, methicillin, penicillin G, ampicillin, carbenicillin, piperacillin and also for cefazolin and nitrocefin, oxyimino cephalosporins and aztreonam (Franceschini et al. 2001). Furthermore nine acetyltransferase genes,

Carbenicillin Mechanism

Figure 1.6. Circular map of the plasmid present in strain Paris (A) and strain Lens (B). For both plasmids from the outside—circle 1: Plasmid genes on the + and - strands, respectively. Black, Tra-region (traTLEKBVC, trbI, traWU, trbC, traNF, trbB, traHGDI); circle 2: G/C bias (G + C/G-C); 3: G + C content of the plasmid with <32.5% G + C short length, between 32.5% and 44.1% medium length and with >44.1% G + C in long lines.

Plasmid Map

Figure 1.6. Circular map of the plasmid present in strain Paris (A) and strain Lens (B). For both plasmids from the outside—circle 1: Plasmid genes on the + and - strands, respectively. Black, Tra-region (traTLEKBVC, trbI, traWU, trbC, traNF, trbB, traHGDI); circle 2: G/C bias (G + C/G-C); 3: G + C content of the plasmid with <32.5% G + C short length, between 32.5% and 44.1% medium length and with >44.1% G + C in long lines.

two of which belonging to the GCN5-related N-acetyltransferase (GNAT) superfamily, are present which could either be implicated in detoxification, drug resistance or gene regulation. Interestingly, 6 kb of the Paris plasmid contain seven genes coding for proteins with 91-98% protein similarity to proteins coded by a plasmid previously identified in a L. longbeachae isolate. Most of the proteins code for unknown functions, but two code for a two-component system, lrpR/lskS, that was shown to be implicated in virulence of this species (Doyle and Heuzenroeder 2002). In contrast, the plasmid identified in strain Lens contains mainly genes coding for proteins of unknown functions.

The role of specific plasmids in virulence of Legionella has not clearly been demonstrated, but a correlation between virulence in a mouse model and the presence of plasmids (Bezanson et al. 1994) and a higher persistence in the environment of strains containing plasmids (Brown et al. 1982) has been reported. Transfer of plasmids from a L. pneumophila donor to another L. pneumophila isolate or another Legionella species may be mediated by the Dot/Icm type-IV secretion system (Vogel et al. 1998). The identification, characterization and analysis of the functional role of Legionella plasmids might add to the understanding of versatility, adaptation and virulence mechanisms of this pathogen.

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