PPE37 is essential for Mycobacterium tuberculosis heme-iron acquisition (HIA) and a defective PPE37 in Mycobacterium bovis BCG prevents HIA.

Mycobacterium tuberculosis (Mtb), one of the world's leading causes of death, must acquire nutrients, such as iron, from the host to multiply and cause disease. Iron is an essential metal and Mtb possesses two different systems to acquire iron from its environment: Siderophore-Mediated Iron Acquisition (SMIA) and Heme-Iron Acquisition (HIA), involving uptake and degradation of heme to release ferrous iron. We have discovered that Mycobacterium bovis BCG, the tuberculosis vaccine strain, is severely deficient in HIA, and exploited this phenotypic difference between BCG and Mtb to identify genes involved in HIA by complementing BCG's defect with a fosmid library. We identified ppe37 , an iron-regulated PPE family gene, as being essential for HIA. BCG complemented with Mtb ppe37 exhibits HIA as efficient as Mtb, achieving robust growth with <0.2 µM hemin. Conversely, deletion of ppe37 from Mtb results in a strain severely attenuated in HIA, with a phenotype nearly identical to BCG, requiring a 200-fold higher concentration of hemin to achieve growth equivalent to its parental strain. A nine amino acid deletion near the N-terminus of BCG PPE37 (amino acids 31 to 39 of the Mtb PPE37 protein) underlies BCG's profound defect in HIA. Significant genetic variability exists in ppe37 genes across different Mtb strains, with more than 60% of sequences from completely sequenced Mtb genomes having mutations that result in altered PPE37 proteins; furthermore, these altered PPE37 proteins are non-functional in HIA. Our findings should allow delineation of the relative roles of HIA and SMIA in Mtb pathogenesis.