Relationships between a new type IV secretion system and the icm/dot virulence system of Legionella pneumophila

We describe here a Legionella pneumophila type IV secretion system that is distinct from the previously described icm/dot system. This type IV secreti on system contains 11 genes (lvh ) homologous to genes of other type IV sec retion systems, arranged in a similar manner. The lvh genes were found to b e located on a DNA island with a GC content higher than the L. pneumophila chromosome. In contrast to the icm/dot system that was shown to be required for intracellular growth in HL-60-derived human macrophages and Acanthamoe ba castellanii, the lvh system was found to be dispensable for intracellula r growth in these two hosts. The lvh system was found to be partially requi red for RSF1010 conjugation, a process that was previously shown to be comp letely dependent on several icm/dot genes. However, results obtained from a nalysis of double mutants in the icm/dot genes and the lvh genes revealed t hat lvh genes can substitute for some components of the icm/dot system for RSF1010 conjugation, but not for intracellular growth. These results indica te that components of the icm/dot system and components of the lvh type IV secretion system are able to interact with one another.