Characterization of genetic loci for carbon metabolism and competition for nodule occupancy in Sinorhizobium meliloti
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In agriculture nitrogen fixation by rhizobial inocula is an environmentally and economically beneficial alternative to synthetic fertilization. The effectiveness of rhizobial inocula can be limited by the inability of inoculum strains to compete with indigenous strains for nodule occupancy. Sinorhizobium meliloti fixes nitrogen in a complex symbiotic relationship with legume hosts including the agriculturally important forage Medicago sativa and the model legume Medicago truncatula. The ability to utilize organic compounds has emerged as an important trait for competitiveness for nodule occupancy in S. meliloti and other rhizobia. This thesis describes the use of bacterial genetics to characterize two carbon metabolism loci in S. meliloti. A genetic locus for erythritol catabolism was characterized and shown to encode an ABC transporter that is required for the catabolism of erythritol, adonitol and L-arabitol, as well as the genes for the catabolism of these three polyols. The ability to utilize erythritol was not necessary for the ability to compete for nodule occupancy in S. meliloti, in contrast to Rhizobium leguminosarum. A genetic locus that encodes components of the De Ley-Doudoroff pathway of galactose catabolism was identified and also characterized. The inability to catabolize galactose resulted in an increased ability to compete for nodule occupancy in S. meliloti. Evidence is presented that is consistent with the hypothesis that increased competitiveness resulted from enhanced production of the symbiotic exopolysaccharide succinoglycan. Inferences are drawn that contribute to the broader understanding of rhizobium-legume symbiosis.