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

The ability to invade and grow in macrophages is necessary forMycobacterium tuberculosis to cause disease. We have found aMycobacterium marinum locus of two genes that is required for both invasion and intracellular survival in macrophages. The genes were designatediipA (mycobacterial invasion and intracellular persistence) andiipB . Theiip mutant, which was created by insertion of a kanamycin resistance gene cassette at the 5′ region ofiipA , was completely avirulent to zebra fish. Expression of theM. tuberculosis orthologue ofiipA , Rv1477, fully complemented theiip mutant for infectivity in vivo, as well as for invasion and intracellular persistence in macrophages. In contrast, theiipB orthologue, Rv1478, only partially complemented theiip 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. Theiip mutant exhibited increased susceptibility to antibiotics and lysozyme and failed to fully separate daughter cells in liquid culture, suggesting a role foriip genes in cell wall structure and function. Altogether, these studies demonstrate an essential role for a p60-containing protein, IipA, in the pathogenesis ofM. marinum infection.