Dysregulation of Magnesium Transport Protects Bacillus subtilis against Manganese and Cobalt Intoxication

• Published in: Journal of bacteriology Vol. 202; no. 7
• Main Authors: Pi, Hualiang, Wendel, Brian M, Helmann, John D
• Format: Journal Article
• Language: English
• Published: United States American Society for Microbiology 11.03.2020
• Subjects: Adaptation, Biological; Bacillus subtilis; Bacillus subtilis - genetics; Bacillus subtilis - metabolism; Bacterial Proteins - genetics; Bacterial Proteins - metabolism; Bacteriology; Biological Transport; Cobalt; Cobalt - metabolism; Depletion; Disruption; Efflux; Enzymes; Gene Expression Regulation, Bacterial; Genetic suppression; Heavy metals; Homeostasis; Intoxication; Iron; Iron - metabolism; Listeria; Listeria monocytogenes; Magnesium; Magnesium - metabolism; Manganese; Manganese - metabolism; Metal ions; Models, Biological; Mutation; Operon; Sensitivity; Transition metals; Transposon mutagenesis; cobalt toxicity; Bacillus subtilis; metal homeostasis; manganese intoxication; magnesium efflux; manganese toxicity; cobalt intoxication
• ISSN: 0021-9193; 1098-5530
• DOI: 10.1128/JB.00711-19

Transition metals are essential for life but are toxic when in excess. Metal ion intoxication may result from the mismetallation of essential metal-dependent enzymes with a noncognate metal. To begin to identify enzymes and processes that are susceptible to mismetallation, we have selected for strains with increased resistance to Mn(II) and Co(II). In , cells lacking the MntR metalloregulator are exquisitely sensitive to Mn(II) but can easily become resistant by acquiring mutations affecting the MntH Mn(II) importer. Using transposon mutagenesis, and starting with an strain, we recovered insertions that inactivated the gene encoding a putative Mg(II) efflux system. Loss of MpfA leads to elevated intracellular Mg(II), increased sensitivity to high Mg(II), and reduced Mn(II) sensitivity. Consistently, we also recovered an insertion disrupting the riboswitch, which normally restricts expression of the major Mg(II) importer. These results suggest that Mn(II) intoxication results from disruption of a Mg(II)-dependent enzyme or process. Mutations that inactivate MpfA were also recovered in a selection for Co(II) resistance beginning with sensitized strains lacking the major Co(II) efflux pump, CzcD. Since both Mn(II) and Co(II) may mismetallate iron-dependent enzymes, we repeated the selections under conditions of iron depletion imposed by expression of the FrvA iron exporter. Under conditions of iron depletion, a wider variety of suppressor mutations were recovered, but they still point to a central role for Mg(II) in maintaining metal ion homeostasis. Cellular metal ion homeostasis is tightly regulated. When metal ion levels are imbalanced, or when one metal is at toxic levels, enzymes may bind to the wrong metal cofactor. Enzyme mismetallation can impair metabolism, lead to new and deleterious reactions, and cause cell death. Beginning with strains genetically sensitized to metal intoxication through loss of efflux or by lowering intracellular iron, we identified mutations that suppress the deleterious effects of excess Mn(II) or Co(II). For both metals, mutations in , encoding a Mg(II) efflux pump, suppressed toxicity. These mutant strains have elevated intracellular Mg(II), suggesting that Mg(II)-dependent processes are very sensitive to disruption by transition metals.