THE LEGIONELLA-PNEUMOPHILA-ICM LOCUS - A SET OF GENES REQUIRED FOR INTRACELLULAR MULTIPLICATION IN HUMAN MACROPHAGES

Legionella pneumophila, the causative agent of Legionnaires' disease a nd related pneumonias, infects, replicates within and eventually kills human macrophages. A key feature of the intracellular lifestyle is th e ability of the organism to replicate within a specialized phagosome which does not fuse with lysosomes or acidify. Avirulent mutants that are defective in intracellular multiplication and host-cell killing ar e unable to prevent phagosome-lysosome fusion. In a previous study, a 12 kb fragment of the L. pneumophila genome containing the icm locus ( intracellular multiplication) was found to enable the mutant bacteria to prevent phagosome-lysosome fusion, to multiply intracellularly and to kill human macrophages. The complemented mutant also regained the a bility to produce lethal pneumonia in guinea-pigs. In order to gain in formation about how L. pneumophila prevents phagosome-lysosome fusion and alters other intracellular events, we have studied the region cont aining the icm locus. This locus contains four genes, icmWXYZ, which a ppear to be transcribed from a single promoter to produce a 2.1-2.4 kb mRNA. The deduced amino, acid sequences of the Icm proteins do not ex hibit significant similarity to other proteins of known sequence, sugg esting that they may carry out novel functions. The icmX gene encodes a product with an apparent signal sequence suggesting that it is a sec reted protein. The icmWXYZ genes are located adjacent to and on the op posite strand from the dot gene, which is also required for intracellu lar multiplication and the ability of L. pneumophila to modify organel le traffic in human macrophages. Five L. pneumophila Icm mutants that had causative agent of been generated with transposon Tn903dIIlacZ wer e found to have inserted the transposon within the icmX, icmY, icmZ an d dot genes, confirming their role in the ability of the organism to m ultiply intracellularly.