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A Mycobacterial Operon Essential for Virulence In Vivo and Invasion and Intracellular Persistence in Macrophages

Program in Microbial Pathogenesis and Host Defense, University of California, San Francisco, California 94143,¹ Department of Cell Biology and Molecular Genetics, University of Maryland, College Park, Maryland 20742,² Departments of Medicine and Microbiology and Immunology, University of California, San Francisco, California 94143³

Received 21 July 2005/ Returned for modification 23 September 2005/ Accepted 26 November 2005

The ability to invade and grow in macrophages is necessary for Mycobacterium tuberculosis to cause disease. We have found a Mycobacterium marinum locus of two genes that is required for both invasion and intracellular survival in macrophages. The genes were designated iipA (mycobacterial invasion and intracellular persistence) and iipB. The iip mutant, which was created by insertion of a kanamycin resistance gene cassette at the 5' region of iipA, was completely avirulent to zebra fish. Expression of the M. tuberculosis orthologue of iipA, Rv1477, fully complemented the iip mutant for infectivity in vivo, as well as for invasion and intracellular persistence in macrophages. In contrast, the iipB orthologue, Rv1478, only partially complemented the iip mutant in vivo and restored invasion but not intracellular growth in macrophages. While IipA and IipB differ at their N termini, they are highly similar throughout their C-terminal NLPC_p60 domains. The p60 domain of Rv1478 is fully functional to replace that of Rv1477, suggesting that the N-terminal sequence of Rv1477 is required for full virulence in vivo and in macrophages. Further mutations demonstrated that both Arg-Gly-Asp (RGD) and Asp-Cys-Ser-Gly (DCSG) sequences in the p60 domain are required for function. The iip mutant exhibited increased susceptibility to antibiotics and lysozyme and failed to fully separate daughter cells in liquid culture, suggesting a role for iip genes in cell wall structure and function. Altogether, these studies demonstrate an essential role for a p60-containing protein, IipA, in the pathogenesis of M. marinum infection.

Editor: J. T. Barbieri

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