Part of the ABC transporter complex IrtAB involved in the import of iron-bound mycobactin (Fe-MBT) and carboxymycobactin (Fe-cMBT) (PubMed:16385031, PubMed:19948799) (By similarity). Transmembrane domains (TMD) form a pore in the membrane and the ATP-binding domain (NBD) is responsible for energy generation (By similarity). Required for replication in human macrophages and in mouse lungs (PubMed:16385031). Forms a heterodimer with IrtA. Repressed by iron and IdeR. In IrtB the ATP-binding domain (NBD) and the transmembrane domain (TMD) are fused. Mutants show normal growth in iron-sufficient conditions, but show a growth defect under iron-deficient conditions. Belongs to the ABC transporter superfamily. Siderophore-Fe(3+) uptake transporter (SIUT) (TC 3.A.1.21) family. PubMed:18461140 reports that IrtB forms a siderophore importer with Rv2895c, however this activity could be due to functional differences of IrtB in the molecular context of M.smegmatis and M.tuberculosis.