L. pneumophila contains a putative type I secretion system encoded by the lssXY-ZABD locus, which might, as in other bacterial pathogens, mediate transport of toxins, degradative enzymes, or other virulence factors across both bacterial membranes. It seems to be specific to L. pneumophila strains (Jacobi and Heuner 2003). The lssXYZABD locus is highly conserved between the three sequenced strains as well as in L. pneumophila strain Corby (94-98% DNA identity; 95-99% protein identity). The downstream lssE gene, encoding a putative signaling protein, is more variable (75-96% DNA identity; 70-94% protein identity). In most type-I secretion systems target protein-encoding genes are located upstream of the secretion machinery itself, but no putative substrate was identified in L. pneumophila to date. However, a candidate substrate could be rtxA, which encodes an Rtx toxin involved in entry and replication in protozoa and human macrophages (Cirillo et al. 2001, 2002). Interestingly the rtxA gene is highly variable in the three sequenced genomes and not adjacent to the lssXYZABD locus.
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