Mycobacterium tuberculosis Expresses a Novel Ph-Dependent Divalent Cation Transporter Belonging to the Nramp Family

Mammalian natural resistance–associated macrophage protein (Nramp) homologues are important determinants of susceptibility to infection by diverse intracellular pathogens including mycobacteria. Eukaryotic Nramp homologues transport divalent cations such as Fe²⁺, Mn²⁺, Zn²⁺, and Cu²⁺. Mycobacterium tuberculosis and Mycobacterium bovis (bacillus Calmette-Guérin [BCG]) also encode an Nramp homologue (Mramp).

RNA encoding Mramp induces ∼20-fold increases in ⁶⁵Zn²⁺ and ⁵⁵Fe²⁺ uptake when injected into Xenopus laevis oocytes. Transport is dependent on acidic extracellular pH and is maximal between pH 5.5 and 6.5. Mramp-mediated ⁶⁵Zn²⁺ and ⁵⁵Fe²⁺ transport is abolished by an excess of Mn²⁺ and Cu²⁺, confirming that Mramp interacts with a broad range of divalent transition metal cations.

Using semiquantitative reverse transcription PCR, we show that Mramp mRNA levels in M. tuberculosis are upregulated in response to increases in ambient Fe²⁺ and Cu²⁺ between <1 and 5 μM concentrations and that this upregulation occurs in parallel with mRNA for y39, a putative metal-transporting P-type ATPase. Using a quantitative ratiometric PCR technique, we demonstrate a fourfold decrease in Mramp/y39 mRNA ratios from organisms grown in 5–70 μM Cu²⁺. M. bovis BCG cultured axenically and within THP-1 cells also expresses mRNA encoding Mramp.

Mramp exemplifies a novel prokaryotic class of metal ion transporter. Within phagosomes, Mramp and Nramp1 may compete for the same divalent cations, with implications for intracellular survival of mycobacteria.