PPE37 Is Essential forMycobacterium tuberculosisHeme-Iron Acquisition (HIA), and a Defective PPE37 inMycobacterium bovisBCG Prevents HIA
ABSTRACTMycobacterium tuberculosis, 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 andM. tuberculosispossesses 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 thatMycobacterium bovisBCG, the tuberculosis vaccine strain, is severely deficient in HIA, and we exploited this phenotypic difference between BCG andM. tuberculosisto identify genes involved in HIA by complementing BCG’s defect with a fosmid library. We identifiedppe37, an iron-regulated PPE family gene, as being essential for HIA. BCG complemented withM. tuberculosisppe37exhibits HIA as efficient as that ofM. tuberculosis, achieving robust growth with <0.2 µM hemin. Conversely, deletion ofppe37fromM. tuberculosisresults in a strain severely attenuated in HIA, with a phenotype nearly identical to that of BCG, requiring a 200-fold higher concentration of hemin to achieve growth equivalent to that of its parental strain. A nine-amino-acid deletion near the N terminus of BCG PPE37 (amino acids 31 to 39 of theM. tuberculosisPPE37 protein) underlies BCG’s profound defect in HIA. Significant genetic variability exists inppe37genes across differentM. tuberculosisstrains, with more than 60% of sequences from completely sequencedM. tuberculosisgenomes having mutations that result in altered PPE37 proteins; furthermore, these altered PPE37 proteins are nonfunctional in HIA. Our findings should allow delineation of the relative roles of HIA and SMIA inM. tuberculosispathogenesis.