Identification and characterization of a new conjugation/type IVA secretion system (trb/tra) of Legionella pneumophila Corby localized on two mobile genomic islands

Horizontal gene transfer probably contributes to evolution of Legionella pneumophila and its adaptation to different environments. Although horizontal gene transfer was observed in Legionella, the mechanism is still not specified. In this study we identified and analysed a new type of conjugation/type IVA secretion system (trb/tra) of L. pneumophila Corby, a virulent human isolate. Two similar versions of this conjugation system were identified, localized on two different genomic islands (Trb-1, 42,710bp and Trb-2, 34,434bp). Trb-1 and Trb-2 are integrated within the tRNA Pro gene Qpc2778) and the tmRNA gene (1pc0164), respectively. Both islands exhibit an oriT region and both can be excised from the chromosome forming episomal circles. Trb-1 was analysed in more detail. It is active and can be horizontally transferred to other Legionella strains by conjugation and then integrated into the genome in a site-specific manner within the tRNA Pro gene. We characterized the sequence of the excision and integration sites of Trb-1 in three different L. pneumophila strains. Here we demonstrate that L. pneumophila exhibits a functional oriT region and that genomic islands in Legionella can be mobilized and conjugated to other species of Legionella. Thus, we describe for the first time a mechanism that may explain the observed horizontal transfer of chromosomal DNA in Legionella. (C) 2007 Elsevier GmbH. All rights reserved.