Abstract
The unique ability of the tuberculosis (TB) bacillus, Mycobacterium tuberculosis, to persist for long periods of time in lung hypoxic lesions chiefly contributes to the global burden of latent TB. We and others previously reported that the M. tuberculosis ancestor underwent massive episodes of horizontal gene transfer (HGT), mostly from environmental species. Here, we sought to explore whether such ancient HGT played a part in M. tuberculosis evolution towards pathogenicity. We were interested by a HGT-acquired M. tuberculosis-specific gene set, namely moaA1-D1, which is involved in the biosynthesis of the molybdenum cofactor. Horizontal acquisition of this gene set was striking because homologues of these moa genes are present all across the Mycobacterium genus, including in M. tuberculosis. Here, we discovered that, unlike their paralogues, the moaA1-D1 genes are strongly induced under hypoxia. In vitro, a M. tuberculosis moaA1-D1-null mutant has an impaired ability to respire nitrate, to enter dormancy and to survive in oxygen-limiting conditions. Conversely, heterologous expression of moaA1-D1 in the phylogenetically closest non-TB mycobacterium, Mycobacterium kansasii, which lacks these genes, improves its capacity to respire nitrate and grants it with a marked ability to survive oxygen depletion. In vivo, the M. tuberculosis moaA1-D1-null mutant shows impaired survival in hypoxic granulomas in C3HeB/FeJ mice, but not in normoxic lesions in C57BL/6 animals. Collectively, our results identify a novel pathway required for M. tuberculosis resistance to host-imposed stress, namely hypoxia, and provide evidence that ancient HGT bolstered M. tuberculosis evolution from an environmental species towards a pervasive human-adapted pathogen.
Highlights
Mycobacterium tuberculosis (Mtb) is an obligate, strictly human-adapted pathogen of major public health importance [1]
We and others previously identified molybdenum cofactor (Moco)-1 as a genomic island acquired through horizontal gene transfer (HGT) after the divergence from the phylogenetically closest nontuberculous mycobacterial species, Mycobacterium marinum and Mycobacterium kansasii [9,11,12]
The genomic signature pattern, i.e. the tetranucleotide frequency [24], of the first ~4-kb fragment of Moco-1, i.e. rv3108-13, which includes the moaA1-D1 genes, is incongruent with that of the Mtb genome as a whole (Fig 1A–1C). This suggests that i/ Moco1 is a mosaic resulting from two or more HGT events, or that ii/ Moco-1 was acquired in a single HGT event followed by compositional amelioration/erosion of the rest of the island, i.e. rv3114-26c (Fig 1A)
Summary
Mycobacterium tuberculosis (Mtb) is an obligate, strictly human-adapted pathogen of major public health importance [1]. The human granuloma is an acidic, nutrient-poor and highly hypoxic environment [3]. To survive such hostile conditions, Mtb is thought to have evolved multiple metabolic mechanisms, including the use of fatty acids as carbon and energy sources for example [4]. How these mechanisms were acquired remains largely unknown
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