A nonribosomal peptide synthase gene driving virulence in Mycobacterium tuberculosis

Nonribosomal peptide synthases produce short peptides in a manner that is distinct from classical mRNA-dependent ribosome-mediated translation. The Mycobacterium tuberculosis genome harbors a nonribosomal peptide synthase gene, nrp, which is part of a gene cluster proposed to be involved in the biosynthesis of isonitrile lipopeptides. Orthologous clusters are found in other slow-growing pathogenic mycobacteria and actinomycetes. To probe the role of the nrp gene in infection, we generated an nrp deletion mutant in M. tuberculosis H37Rv and tested its virulence in immunocompetent (C57BL/6) mice. The nrp mutant strain displayed lower initial growth rates in the lungs and a defective dissemination to the spleens of infected mice. Mice infected with the mutant strain also survived for twice as long as those infected with wild-type M. tuberculosis and, remarkably, showed subdued pathology, despite similar bacterial loads at later stages of infection. The differences in the course of infection between wild-type and nrp mutant strains were accompanied by distinct dynamics of the immune response. Most strikingly, the nrp mutant was highly attenuated in immunodeficient (SCID-, recombination activating 2 [RAG2]-, and gamma interferon [IFN-γ]-deficient) mice, suggesting that macrophages control the nrp mutant more efficiently than they control the wild-type strain. However, in the presence of IFN-γ, both strains were equally controlled. We propose that the nrp gene and its associated cluster are drivers of virulence during the early stages of infection.