Genome Rearrangements and Horizontal Gene Transfer in Legionella pneumophila

Legionella pneumophila is found in diverse ecological niches such as axenic cultures, biofilms with other microbes, and intracellular vacuoles of protozoa and human cells. This may have provided ample opportunities for the organisms to rearrange their genomes and to accumulate genes that help them survive in these different environments. Legionella's ability to survive in harsh conditions such as in plumbing systems treated with potent biocides may require additional genes necessary for detoxification. The needed gene pool can be assembled via gene family expansions and horizontal gene transfer from other organisms. Genome comparisons of the Philadelphia 1 strain and those of Paris and Lens, indicating positions of the large-scale genome rearrangements. Phylogenetic and gene composition analysis indicate that different L. pneumophila strains display distinct acquisition histories for these gene subsets, suggesting gene rearrangements as well as repeated horizontal gene transfer events. Several dozen eukaryotic gene homologs with distinct phenotypes which appear to be acquired by gene transfer were identified. Overall, the genome of L. pneumophila reveals features supporting genome rearrangement events of different scales. Given its intracellular existence, it is not surprising that examination of the genome sequence offers evidence that L. pneumophila can be an active participant in horizontal gene transfer. As more strains of L. pneumophila are sequenced, along with other species of Legionella, much more will be learned about their evolutionary origins, lifestyle, and potential for pathogenicity.