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Copper is an important element in host–microbe interactions, acting both as a catalyst in enzymes and as a potential toxin. Cu+-ATPases drive cytoplasmic Cu+ efflux and protect bacteria against metal overload. Many pathogenic and symbiotic bacteria contain multiple Cu+-ATPase genes within particular genetic environments, suggesting alternative roles for each resulting protein. This hypothesis was tested by characterizing five homologous Cu+-ATPases present in the symbiotic organism Sinorhizobium meliloti. Mutation of each gene led to different phenotypes and abnormal nodule development in the alfalfa host. Distinct responses were detected in free-living S. meliloti mutant strains exposed to metal and redox stresses. Differential gene expression was detected under Cu+, oxygen or nitrosative stress. These observations suggest that CopA1a maintains the cytoplasmic Cu+ quota and its expression is controlled by Cu+ levels. CopA1b is also regulated by Cu+ concentrations and is required during symbiosis for bacteroid maturation. CopA2-like proteins, FixI1 and FixI2, are necessary for the assembly of two different cytochrome c oxidases at different stages of bacterial life. CopA3 is a phylogenetically distinct Cu+-ATPase that does not contribute to Cu+ tolerance. It is regulated by redox stress and required during symbiosis. We postulated a model where non-redundant homologous Cu+-ATPases, operating under distinct regulation, transport Cu+ to different target proteins.
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