Sugar alcohol metabolism in the legume endosymbiont Sinorhizobium meliloti
Symbiotic nitrogen fixation by diazotrophic bacteria is an environmentally benign alternative to synthetic fertilizer application to agricultural fields. The effectiveness of inoculum strains can be reduced by competition with native strains present in the soil. Sinorhizobium meliloti is a soil bacterium that can establish a nitrogen fixing symbiosis with the forage crop Medicago sativa and the model legume Medicago truncatula. The ability to utilize organic compounds has been shown to be important for competition for nodule occupancy in S. meliloti and other symbiotic bacteria. Here, genetic and biochemical techniques are used to examine the metabolism of several substrates in the model organism S. meliloti. Genetic loci involved in sugar alcohols galactitol as well as sorbitol, mannitol, and D-arabitol were identified and characterized. Special attention was paid to sorbitol dehydrogenase SmoS and a crystal structure with a resolution of 2.0 Å was generated. These experiments led to the characterization of the metabolism of D-arabinose and L-fucose, which is carried out in part by loci that are involved in mannitol and D-arabitol utilization. Additionally, while S. meliloti cannot utilize the sugar alcohol xylitol, it was observed that spontaneous mutations would arise at a modest frequency that permitted growth on xylitol. This mutation increases the metabolic capacity of the organism, and was genetically characterized and determined to be related to the movement of an ISRm2011-2 insertion sequence element. These results contribute to our understanding of the metabolism of the model organism S. meliloti and further efforts to engineer symbiosis in other organisms.
Sinorhizobium meliloti, Metabolism, Symbiosis, Nitrogen fixation, Rhizobia, Medicago, Sugar alcohols