Regulation, Integrase-Dependent Excision, and Horizontal Transfer of Genomic Islands in Legionella pneumophila

• Published in: Journal of Bacteriology Vol. 195; no. 7; pp. 1583 - 1597
• Main Authors: Lautner, Monika, Schunder, Eva, Herrmann, Vroni, Heuner, Klaus
• Format: Journal Article
• Language: English
• Published: United States American Society for Microbiology 01.04.2013
• Subjects: Acanthamoeba castellanii; Acanthamoeba castellanii - microbiology; Bacterial Secretion Systems - genetics; Bacteriology; Chromosomes, Bacterial; coculture; Conjugation, Genetic; Gene Transfer, Horizontal; Genes; Genomic Islands; Genomics; Gram-negative bacteria; Humans; Integrases - metabolism; Legionella pneumophila; Legionella pneumophila - genetics; Legionella pneumophila - metabolism; Legionnaires disease; macrophages; mutants; Mutation; Plasmids; Recombination, Genetic; secretion; vacuoles
• ISSN: 0021-9193; 1098-5530; 1067-8832
• DOI: 10.1128/JB.01739-12

Legionella pneumophila is a Gram-negative freshwater agent which multiplies in specialized nutrient-rich vacuoles of amoebae. When replicating in human alveolar macrophages, Legionella can cause Legionnaires' disease. Recently, we identified a new type of conjugation/type IVA secretion system (T4ASS) in L. pneumophila Corby (named trb-tra). Analogous versions of trb-tra are localized on the genomic islands Trb-1 and Trb-2. Both can exist as an episomal circular form, and Trb-1 can be transferred horizontally to other Legionella strains by conjugation. In our current work, we discovered the importance of a site-specific integrase (Int-1, lpc2818) for the excision and conjugation process of Trb-1. Furthermore, we identified the genes lvrRABC (lpc2813 to lpc2816) to be involved in the regulation of Trb-1 excision. In addition, we demonstrated for the first time that a Legionella genomic island (LGI) of L. pneumophila Corby (LpcGI-2) encodes a functional type IV secretion system. The island can be transferred horizontally by conjugation and is integrated site specifically into the genome of the transconjugants. LpcGI-2 generates three different episomal forms. The predominant episomal form, form A, is generated integrase dependently (Lpc1833) and transferred by conjugation in a pilT-dependent manner. Therefore, the genomic islands Trb-1 and LpcGI-2 should be classified as integrative and conjugative elements (ICEs). Coculture studies of L. pneumophila wild-type and mutant strains revealed that the int-1 and lvrRABC genes (located on Trb-1) as well as lpc1833 and pilT (located on LpcGI-2) do not influence the in vivo fitness of L. pneumophila in Acanthamoeba castellanii.