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

Abstract

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 Fe(2+), Mn(2+), Zn(2+), and Cu(2+). Mycobacterium tuberculosis and Mycobacterium bovis (bacillus Calmette-Guérin [BCG]) also encode an Nramp homologue (Mramp). RNA encoding Mramp induces approximately 20-fold increases in (65)Zn(2+) and (55)Fe(2+) 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 (65)Zn(2+) and (55)Fe(2+) transport is abolished by an excess of Mn(2+) and Cu(2+), 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 Fe(2+) and Cu(2+) between <1 and 5 microM 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 microM Cu(2+). 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.