Regulation of Heme Biosynthesis in the Bradyrhizobium japonicum/Soybean Symbiosis

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The regulation of heme biosynthesis in nitrogen-fixing soybean root nodules is being studied as a means of understanding how the bacterial symbiont Bradyrhizobium japonicum interacts with the plant to regulate metabolic processes essential for symbiosis. Bacterial heme is believed to be the prosthetic group of the plant nodule protein leghemoglobin, hence heme and apoprotein synthesis by the respective orgainism must be coordinated during leghemoglobin formation. Experiments will elucidate the exact role of the B. japonicum heme biosynthetic pathway in leghemoglobin formation, and deterine how heme synthesis genes are regulated during nodule development Specific objectives are to (i) isolate B. japonicum heme synthesis mutants, (ii) determine the effects of the bacterial heme synthesis lesions on leghemoglobin synthesis, (iii) isolate and characterize B. japonicum heme synthesis genes, and (iv) study the regulation of heme synthesis genes by nodule-associated factors and by bacterial regulatory protein. Finally, the regulation of bacterial heme synthesis will be correlated with leghemoglobin formation. Biological nitrogen fixation is important because it is an inexpensive and environmentally-sound source of nitrogen fertilizer. Understanding plant-microbe interactions at the molecular level is important in any attempt to improve the symbiosis or to extend it to other plants by genetic engineering. The study of the Bradyrhizobium japonicum/soybean interaction will provide a better understanding of interkingdom commmunication as well as contribute to the body of knowledge of heme biosynthesis in general, of which there is surprisingly little genetic information considering the ubiquity of this pathway in nature.