Heme Biosynthesis in the Bradyrhizobium Japonicum/Soybean Symbiosis
Mark O'Brian Principal Investigator
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9722974 O 'Brian Bradyrhizobium japonicum is the bacterial endosymbiont of soybean that fixes nitrogen within cells of a specialized plant organ called a root nodule. The broad objective of the research is to understand the regulation and coordination of processes necessary for symbiotic development and function. Nitrogen fixation is energy-intensive, and thus changes in the plant and bacterial heme profile are concomitant with nodule ontogeny to accommodate this demand. The project addresses the hypothesis that heme biosynthesis is a regulated, interactive process in B. japonicum, and that it is coordinated with cell differentiation and function. In addition, this system is excellent for studies of heme synthesis and heme protein function, and serves as a paradigm for both prokaryotes and eukaryotes. (-aminolevulinic acid (ALA) is the first universal committed heme precursor, and work from the P.I. 's laboratory indicates that B. japonicum heme can be synthesized by host-derived ALA. This model requires a mechanism for acquiring exogenous ALA, and suggests regulation of bacterial heme biosynthesis at steps other than, or in addition to, ALA synthesis. Indeed, data show that B. japonicum hemB is a highly regulated gene. A novel protein called IRR has been identified that mediates iron control of heme synthesis directly by repressing hemB expression and indirectly by activating iron transport; hemB and IRR function and regulation will be studied. The ALA uptake system of B. japonicum has been partially characterized, and shown to be controlled by a homolog of the transcriptional regulator Lrp. Finally, recent findings question the long-held view that plant nodule hemoglobin heme is derived by the rhizobial symbiont, and that problem will be revisited. Certain bacteria that live in the soil can form a beneficial relationship with soybeans and other related plants of agricultural interest. The relationship confers on the plant the ability to grow in soil in the absence of applied nitrogen fe rtilizer, thus it has economic and environmental importance. In addition, the symbiosis has become a good model for understanding other biological phenomena that appear to differ from the one under study, but in fact are very similar at the molecular level. The research focuses on understanding how the bacterium Bradyrhizobium japonicum changes during its interaction with soybean to adapt to its role in symbiosis, and how formation of heme, a compound important in energy metabolism, is controlled.