The mntH gene encodes the major Mn2+ transporter in Bradyrhizobium japonicum and is regulated by manganese via the Fur protein

Abstract

The bacterial Nramp family protein MntH is a divalent metal transporter, but mntH mutants have little or no phenotype in organisms where it has been studied. Here, we identify the mntH homologue of Bradyrhizobium japonicum, and demonstrate that it is essential for Mn(2+) transport and for maintenance of cellular manganese homeostasis. Transport activity was induced under manganese deficiency, and Fe(2+) did not compete with (54)Mn(2+) for uptake by cells. The steady-state level of mntH mRNA was negatively regulated by manganese, but was unaffected by iron. Control of mntH expression and Mn(2+) transport by manganese was lost in a fur strain, resulting in constitutively high activity. Fur protected a 35 bp region of the mntH promoter in DNase I footprinting analysis that includes three imperfect direct repeat hexamers that are needed for full occupancy. Mn(2+) increased the affinity of Fur for the mntH promoter by over 50-fold, with a K(d) value of 2.2 nM in the presence of metal. The findings identify MntH as the major Mn(2+) transporter in B. japonicum, and show that Fur is a manganese-responsive regulator in that organism. Furthermore, Fe(2+) is neither a substrate for MntH nor a regulator of mntH expression in vivo.