Heme Biosynthesis in the Bradyrhizobium japonicum/Soybean Symbiosis

9404818大豆根瘤菌是大豆中的细菌内共生菌，在一种被称为根瘤的特殊植物器官的细胞内起着固氮细胞器的作用。这两种共生体之间发生了许多相互作用，促进了结核的形成、发展和维持。这项工作的广泛目标是了解豆科植物如何与它们的细菌伙伴沟通，以调节共生所必需的代谢过程。特别是，我们正在阐明日本双胞菌和大豆之间的相互作用事件，这些事件控制着根瘤中血红素的合成。随着各自的细胞类型在根瘤中分化，两种生物对血红素的需求都增加了，相应的血红素蛋白是共生固氮的基本特征。初步工作表明，植物寄主直接参与了根瘤中细菌血红素的合成，其他数据表明，大豆血红素的来源是日本芽孢杆菌。上述两个假说正在通过以下方式得到验证：1)构建和鉴定在摄取血红素前体-氨基酮丙酸(ALA)方面存在缺陷的日本芽孢杆菌突变体。大豆ALA有可能在根瘤中合成细菌血红素，这一机制的可行性将通过ALA吸收突变体的获得和鉴定来检验。Ii)从植物ALA合成的细菌血红素推断，B.Japan Ala脱水酶是根瘤中形成血红素的第一个必需的细菌酶，对突变体的分析证实了这一预测。因此，对血红蛋白B的调控。将对编码丙氨酸脱水酶的基因进行检测。Iii)通过构建和鉴定温度敏感的铁络合酶突变体，确定日本双胞菌是否能够提供大豆豆红蛋白的血红素修复基团。突变体将允许在允许的温度下正常发育根瘤，但将允许在这些结节中灭活细菌的血红素合成酶。这项工作将增加我们对大豆和其他豆类作物生长的重要一步的理解。***

9404818 O'Brian Bradyrhizobium japonicum is the bacterial endosymbiont of soybean (Glycine max) that functions as a nitrogen-fixing organelle within cells of a specialized plant organ called a root nodule. Many interactions occur between the two symbionts for the initiation, development and maintenance of nodules. The broad objective of this work is to understand how leguminous plants communicate with their bacterial partners to regulate metabolic processes essential for symbiosis. In particular, we are elucidating the interactive events between B. japonicum and soybean that control heme synthesis in root nodules. The demand for heme increases in both organisms as the respective cell types differentiate in nodules, and the corresponding heme proteins are an essential feature of symbiotic nitrogen fixation. Preliminary work indicates that the plant host is directly involved in bacterial heme synthesis in nodules, and other data have implicated B. japonicum as the source for soybean hemoglobin heme. The two aforesaid hypotheses are being tested by i) Constructing and characterizing B. japonicum mutants defective in uptake of the heme precursor delta-aminolevulinic acid (ALA). It is possible that bacterial heme can be synthesized from soybean-derived ALA in nodules, and the tenability of this mechanism will be tested by the acquisition and characterization of ALA uptake mutants. ii) Bacterial heme synthesis from plant ALA infers that B. japonicum ALA dehydratase is the first essential bacterial enzyme for heme formation in nodules, and analysis of mutants bears out that prediction. Therefore, the regulation of hemB. the gene encoding ALA dehydratase will be examined. iii) Determining whether B. japonicum can provide the heme prosthetic group of soybean leghemoglobin by construction and characterization of a temperature-sensitive ferrochelatase mutant. The mutant will allow normal nodule development at a permissive temperature, but will permit the inactivation of bacterial heme synt hesis in those nodules. %%% This work will increase our understanding of an important step in soybean and other legume growth. ***