Mycobacterium tuberculosis expresses a novel pH-dependent divalent cation transporter belonging to the Nramp family

Mammalian natural resistance-associated macrophage protein (Nramp) homologu es are important determinants of susceptibility to infection by diverse int racellular pathogens including mycobacteria. Eukaryotic Nramp homologues tr ansport divalent cations such as Fe2+, Mn2+, Zn2+, and Cu2+. Mycobacterium tuberculosis and Mycobacterium bovis (bacillus Calmette-Guerin [BCG]) also encode an Nramp homologue (Mramp). RNA encoding Mramp induces similar to 20-fold increases in Zn-65(2+) and Fe -55(2+) uptake when injected into Xenopus laevis oocytes. Transport is depe ndent on acidic extracellular pH and is maximal between pH 5.5 and 6.5. Mra mp-mediated Zn-65(2+) and 55Fe(2+) transport is abolished by an excess of M n2+ and Cu2+, confirming that Mramp interacts with a broad range of divalen t transition metal cations. Using semiquantitative reverse transcription PCR, we show that Mramp mRNA l evels in M. tuberculosis are upregulated in response to increases in ambien t Fe2+ and Cu2+ between <1 and 5 mu M concentrations and that this upregula tion 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 mu M Cu2+. M. bovis BCG cultured axenically and within THP-1 cells also exp resses mRNA encoding Mramp. Mramp exemplifies a novel prokaryotic class of metal ion transporter. Withi n phagosomes, Mramp and Nramp1 may compete for the same divalent cations, w ith implications for intracellular survival of mycobacteria.