Mycobacterium avium subsp.
paratuberculosis is a host-adapted pathogen that evolved from the environmental bacterium M. avium subsp. hominissuis through gene loss and gene acquisition. Growth of M. avium subsp.
paratuberculosis in the laboratory is enhanced by supplementation of the media with the
iron-binding
siderophore mycobactin J. Here we examined the production of
mycobactins by related organisms and searched for an alternative
iron uptake system in M. avium subsp.
paratuberculosis. Through thin-layer chromatography and radiolabeled
iron-uptake studies, we showed that M. avium subsp.
paratuberculosis is impaired for both
mycobactin synthesis and
iron acquisition. Consistent with these observations, we identified several mutations, including deletions, in M. avium subsp.
paratuberculosis genes coding for
mycobactin synthesis. Using a transposon-mediated mutagenesis screen conditional on growth without myobactin, we identified a potential
mycobactin-independent
iron uptake system on a M. avium subsp.
paratuberculosis-specific genomic island, LSP(P)15. We obtained a transposon (Tn) mutant with a disruption in the LSP(P)15 gene MAP3776c for targeted study. The mutant manifests increased
iron uptake as well as intracellular
iron content, with genes downstream of the transposon insertion (MAP3775c to MAP3772c [MAP3775-2c]) upregulated as the result of a polar effect. As an independent confirmation, we observed the same
iron uptake phenotypes by overexpressing MAP3775-2c in wild-type M. avium subsp.
paratuberculosis. These data indicate that the horizontally acquired LSP(P)15 genes contribute to
iron acquisition by M. avium subsp.
paratuberculosis, potentially allowing the subsequent loss of
siderophore production by this pathogen.
IMPORTANCE: Many microbes are able to scavenge
iron from their surroundings by producing
iron-chelating
siderophores. One exception is Mycobacterium avium subsp.
paratuberculosis, a fastidious, slow-growing animal pathogen whose growth needs to be supported by exogenous mycobacterial
siderophore (
mycobactin) in the laboratory. Data presented here demonstrate that, compared to other closely related M. avium subspecies,
mycobactin production and
iron uptake are different in M. avium subsp.
paratuberculosis, and these phenotypes may be caused by numerous deletions in its
mycobactin biosynthesis pathway. Using a genomic approach, supplemented by targeted genetic and biochemical studies, we identified that LSP(P)15, a horizontally acquired genomic island, may encode an alternative
iron uptake system. These findings shed light on the potential physiological consequence of horizontal gene transfer in M. avium subsp.
paratuberculosis evolution.