Iron is an essential nutrient not freely available to microorganisms infecting mammals. To overcome iron deficiency, bacteria have evolved various strategies including the synthesis and secretion of high affinity iron chelators known as siderophores. The siderophores produced and secreted by M. tuberculosis, exomycobactins, compete for iron with host iron binding proteins and together with the iron regulated ABC transporter IrtAB, are required for the survival of M. tuberculosis in iron deficient conditions and for normal replication in macrophages and in mice. This study further characterizes the role of IrtAB in M. tuberculosis iron acquisition. Our results demonstrate a role for IrtAB in iron import and show that the amino terminus domain of IrtA is a flavin-adenine dinucleotide binding domain essential for iron acquisition. These results suggest a model in which the amino terminus of IrtA functions to couple iron transport and assimilation.

Mycobacterium avium subspecies paratuberculosis (MAP), the causative agent of Johne's disease in cattle and sheep, has unique iron requirements in that it is mycobactin dependent for cultivation in-vitro. The iron dependent regulator (IdeR) is a well-characterized global regulator responsible for maintaining iron homeostasis in Mycobacterium tuberculosis (MTB). We identified an orthologous segment in MAP genome, MAP2827, with >93% amino acid identity to MTB IdeR. Electrophoretic mobility shift assays and DNase protection assays confirmed that MAP2827 protein binds the 19 base pair consensus motif (iron box) on the MAP genome. Resequencing MAP2827 from multiple isolates revealed a non-synonymous change (R91G) exclusive to sheep strains. Reporter gene assays and quantitative real time RT-PCR assays in two diverse MAP strains and in an ideR deletion mutant of M. smegmatis (mc2155) suggested that both sheep MAP IdeR (sIdeR) and cattle MAP IdeR (cIdeR) repressed mbtB transcription at high iron concentration and relieved repression at low iron concentration. On the other hand, bfrA (an iron storage gene) was upregulated by cIdeR when presented with MTB or cattle MAP bfrA promoter and was downregulated by sIdeR in the presence of MTB or sheep or cattle MAP bfrA promoters, at high iron concentration. The differential iron regulatory mechanisms between IdeR regulated genes across strains may contribute to the differential growth or pathogenic characteristics of sheep and cattle MAP strains. Taken together, our study provides a possible reason for mycobactin dependency and suggests strong implications in the differential iron acquisition and storage mechanisms in MAP.downregulated by sIdeR in the presence of MTB or sheep or cattle MAP bfrA promoters, at high iron concentration. The differential iron regulatory mechanisms between IdeR regulated genes across strains may contribute to the differential growth or pathogenic characteristics of sheep and cattle MAP strains. Taken together, our study provides a possible reason for mycobactin dependency and suggests strong implications in the differential iron acquisition and storage mechanisms in MAP.

The mycobacterial IdeR protein is a metal-dependent regulator of the DtxR (diphtheria toxin repressor) family. In the presence of iron, it binds to a specific DNA sequence in the promoter regions of the genes that it regulates, thus controlling their transcription. In this study, we provide evidence that ideR is an essential gene in Mycobacterium tuberculosis. ideR cannot normally be disrupted in this mycobacterium in the absence of a second functional copy of the gene. However, a rare ideR mutant was obtained in which the lethal effects of ideR inactivation were alleviated by a second-site suppressor mutation and which exhibited restricted iron assimilation capacity. Studies of this strain and a derivative in which IdeR expression was restored allowed us to identify phenotypic effects resulting from ideR inactivation. Using DNA microarrays, the iron-dependent transcriptional profiles of the wild-type, ideR mutant, and ideR-complemented mutant strains were analyzed, and the genes regulated by iron and IdeR were identified. These genes encode a variety of proteins, including putative transporters, proteins involved in siderophore synthesis and iron storage, members of the PE/PPE family, a membrane protein involved in virulence, transcriptional regulators, and enzymes involved in lipid metabolism.

Tuberculosis continues to kill millions of people around the world. New tools to prevent and treat this disease are urgently needed. Similar to most microorganisms, Mycobacterium tuberculosis - the causative agent of tuberculosis - requires iron for essential metabolic pathways. Because iron is not freely available in the host, pathogens must actively compete for this metal to establish an infection but they must also carefully control iron acquisition as excess free iron can be extremely toxic. Recent studies have demonstrated that failure to assemble the iron acquisition machinery or to repress iron uptake has deleterious effects for M. tuberculosis. Here, we review how M. tuberculosis obtains iron in a regulated manner and discuss how these processes could potentially be disrupted to interfere with the survival and replication of this bacterium in the host.

The role of iron in mycobacteria as in other bacteria goes beyond the need for this essential cofactor. Limitation of this metal triggers an extensive response aimed at increasing iron acquisition while coping with iron deficiency. In contrast, iron-rich environments prompt these prokaryotes to induce synthesis of iron storage molecules and to increase mechanisms of protection against iron-mediated oxidative damage. The response to changes in iron availability is strictly regulated in order to maintain sufficient but not excessive and potentially toxic levels of iron in the cell. This response is also linked to other important processes such as protection against oxidative stress and virulence. In bacteria, iron metabolism is regulated by controlling transcription of genes involved in iron uptake, transport and storage. In mycobacteria, this role is fulfilled by the iron-dependent regulator IdeR. IdeR is an essential protein in Mycobacterium tuberculosis, the causative agent of human tuberculosis. It functions as a repressor of iron acquisition genes, but is also an activator of iron storage genes and a positive regulator of oxidative stress responses.