FixI and FixI2: Homologous proteins with unique functions in Sinorhizobium meliloti

Collins, Jessica M.

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FixI and FixI2: Homologous proteins with unique functions in Sinorhizobium meliloti

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Cu+-ATPases are transmembrane enzymes that couple the efflux of cytoplasmic Cu+ to the hydrolysis of ATP. It is well established that Cu+-ATPases control cytoplasmic Cu+ levels. However, bacterial genomes, particularly those of symbiotic/pathogenic organisms, contain multiple copies of genes encoding Cu+-ATPases, challenging the idea of a singular role for these enzymes. Our lab has demonstrated that one of the two Cu+-ATPases in Pseudomonas aeruginosa, a FixI-type ATPase, has an alternative role, most likely Cu+ loading of cytochrome c oxidase (Cox). To further study alternative roles of Cu+-ATPases, we study the symbiont Sinorhizobium meliloti. Rhizobia are soil-dwelling bacteria that interact with legumes, forming plant root nodules that actively fix N2. The S. meliloti genome contains five Cu+-ATPases, two of which are FixI-type. Both of these enzymes, termed FixI1 and FixI2, are downstream of Cox operons. We hypothesized that the presence of multiple FixI-type ATPases was not an example of redundancy, but rather is an evolutionary adaptation that allows rhizobia to survive under the wide variety of adverse conditions faced during early infection and establishment of symbiosis. Towards this goal, this work focused on examining the effects of mutation of each ATPase on both free-living bacteria and on the ability of rhizobia to establish an effective symbiosis with its host legume. Each of these mutants presents a different phenotype at varying points of the nodulation process, and only the fixI2 mutation produces a respiratory-deficient phenotype during aerobic growth. These results are consistent with our hypothesis that the two proteins have non-redundant physiological functions. Understanding the factors that contribute to an effective symbiosis is beneficial, since N2 fixation in legumes is important to both agriculture and industry.

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