Magnesium-dependent processes are targets of bacterial manganese toxicity

A Bradyrhizobium japonicum mutant defective in the gene encoding the high-affinity Mn²⁺ transporter MntH has a severe growth phenotype under manganese limitation. Here, we isolated suppressor mutants of an mntH strain that grew under manganese limitation, and activities of high-affinity Mn²⁺ transport and Mn²⁺-dependent enzymes were partially rescued. The suppressor strains harbour gain-of-function mutations in the gene encoding the Mg²⁺ channel MgtE. The MgtE variants likely allow Mn²⁺ entry via loss of a gating mechanism that normally holds the transporter in the closed state when cellular Mg²⁺ levels are high. Both MgtE-dependent and MgtE-independent suppressor phenotypes were recapitulated by magnesium-limited growth of the mntH strain. Growth studies of wild-type cells suggest that manganese is toxic to cells when environmental magnesium is low. Moreover, extracellular manganese and magnesium levels were manipulated to inhibit growth without substantially altering the intracellular content of either metal, implying that manganese toxicity depends on its cellular distribution rather than the absolute concentration. Mg²⁺-dependent enzyme activities were found to be inhibited or stimulated by Mn²⁺. We conclude that Mn²⁺ can occupy Mg²⁺ binding sites in cells, and suggest that Mg²⁺-dependent processes are targets of manganese toxicity.