Heme Biosynthesis in the Bradyrhizobium japonicum/Soybean Symbiosis

[404818] O'Brian Bradyrhizobium japonicum是大豆（Glycine max）的细菌内共生体，在称为根瘤的特殊植物器官的细胞内起固氮细胞器的作用。这两种共生体之间发生许多相互作用，以产生、发展和维持结核。这项工作的主要目的是了解豆科植物如何与它们的细菌伙伴交流，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歧杆菌和大豆之间控制根瘤中血红素合成的相互作用事件。两种生物对血红素的需求随着各自细胞类型在结节中的分化而增加，而相应的血红素蛋白是共生固氮的基本特征。初步研究表明，植物寄主直接参与结核中细菌血红素的合成，其他数据表明日本芽孢杆菌是大豆血红素的来源。上述两种假设正在通过i)构建和表征对血红素前体三角洲氨基乙酰丙酸（ALA）摄取缺陷的日本芽孢杆菌突变体来验证。细菌血红素可能是由大豆来源的ALA在根瘤中合成的，这一机制的可行性将通过获取和表征ALA摄取突变体来验证。ii)细菌从植物ALA合成血红素，推断日本芽孢杆菌ALA脱水酶是结核中形成血红素的第一必需细菌酶，突变体分析证实了这一预测。因此，hemB的调控。检测编码ALA脱水酶的基因。iii)通过构建和表征温度敏感的铁螯合酶突变体，确定日本芽孢杆菌是否可以提供大豆血红蛋白的血红素假体基。突变体将允许在允许的温度下正常的结节发育，但将允许这些结节中的细菌血红素合成失活。这项工作将增加我们对大豆和其他豆科植物生长的一个重要步骤的了解。＊＊＊

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. ***