FIG 2.

Site-specific recombination at the dif site is highly efficient and does not require RecA. (A) Schematic representation of the intermolecular recombination assay used to quantify att site-specific recombination in L. pneumophila. A 5.5-kb nonreplicating plasmid modified to include the 22-bp LME-1 att site (or control sequences) is transformed into L. pneumophila strain Lp02. If recombination between the plasmid att site and the chromosomal dif site takes place, the whole plasmid is integrated into the genome and gentamicin resistance is conferred to the cell. Gentamicin-resistant colonies are counted to quantify recombination. (B) Transformation efficiency (relative to that of a replicative plasmid control) of a nonreplicative plasmid containing the 22-bp att site, a 22-bp control sequence, or a 2,200-bp intergenic region of the Lp02 genome, transformed into the wild-type or ΔrecA Lp02 strains. Each dot shows the value obtained from a single experiment, and each horizontal line represents the geometric mean of 3 independent experiments.