Dr. Ivan Oresnik Department of Microbiology, University of Manitoba Relationship between central carbon metabolism and nitrogen fixation in Sinorhizobium meliloti Carbon metabolism is generally well understood in Sinorhizobium meliloti. The literature is consistent with the role of dicarboxylic acid metabolism while the bacteroid is actively fixing nitrogen. However, the literature contains many nitrogen fixation phenotypes ascribed to mutants that encode enzymes in central carbon metabolism that make little sense, or are even paradoxical. For example, a mutation in pckA, which is necessary for gluconeogenesis and encodes phosphoenolpyruvate carboxy kinase, consistently gives nitrogen fixation rates that are approximately 50% of wild-type, yet no measurable enzyme activity can be detected in bacteroids. Similarly, our work has shown strains that do not have triose phosphate isomerase activity also yield plants with 50% dry matter accumulation when grown under nitrogen deficient conditions. To date there is no clear explanation why these lesions affect nitrogen fixation based on our current knowledge. Based on our observations, we are hypothesizing that carbon metabolism may be correlated with endoreplication during bacteroid development and that rates of nitrogen fixation may be linked to the copy number of genes directly involved in nitrogen fixation. Although this may explain what occurs in indeterminate nodules, it probably does not apply to determinate nodules, suggesting that what limits nitrogen fixation between these nodule types may be different. Comments are closed.
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