Delgado M J, Bedmar E J, Downie J A
Departamento de Microbiología del Suelo y Sistemas Simbióticos, Estación Experimental del Zaidin, CSIC, Granada, Spain.
Adv Microb Physiol. 1998;40:191-231. doi: 10.1016/s0065-2911(08)60132-0.
Rhizobia fix nitrogen in a symbiotic association with leguminous plants and this occurs in nodules. A low-oxygen environment is needed for nitrogen fixation, which paradoxically has a requirement for rapid respiration to produce ATP. These conflicting demands are met by control of oxygen flux and production of leghaemoglobin (an oxygen carrier) by the plant, coupled with the expression of a high-affinity oxidase by the nodule bacteria (bacteroids). Many of the bacterial genes encoding cytochrome synthesis and assembly have been identified in a variety of rhizobial strains. Nitrogen-fixing bacteroids use a cytochrome cbb3-type oxidase encoded by the fixNOQP operon; electron transfer to this high-affinity oxidase is via the cytochrome bc1 complex. During free-living growth, electron transport from the cytochrome bc1 complex to cytochrome aa3 occurs via a transmembrane cytochrome c (CycM). In some rhizobia (such as Bradyrhizobium japonicum) there is a second cytochrome oxidase that also requires electron transport via the cytochrome bc1 complex. In parallel with these cytochrome c oxidases there are quinol oxidases that are expressed during free-living growth. A cytochrome bb3 quinol oxidase is thought to be present in B. japonicum; in Rhizobium leguminosarum, Rhizobium etli and Azorhizobium caulinodans cytochrome d-type oxidases have been identified. Spectroscopic data suggest the presence of a cytochrome o-type oxidase in several rhizobia, although the absence of haem O in B. japonicum may indicate that the absorption attributed to cytochrome o could be due to a high-spin cytochrome b in a cytochrome bb3-type oxidase. In some rhizobia, mutation of genes involved in cytochrome c assembly does not strongly affect growth, presumably because the bacteria utilize the cytochrome c-independent quinol oxidases. In this review, we outline the work on various rhizobial mutants affected in different components of the electron transport pathways, and the effects of these mutations on symbiotic nitrogen fixation and free-living growth.
根瘤菌与豆科植物共生固氮，这一过程发生在根瘤中。固氮需要低氧环境，但矛盾的是，固氮过程又需要快速呼吸以产生ATP。植物通过控制氧气通量和产生豆血红蛋白（一种氧气载体），并结合根瘤菌（类菌体）中高亲和力氧化酶的表达，来满足这些相互矛盾的需求。在多种根瘤菌菌株中，已经鉴定出许多编码细胞色素合成和组装的细菌基因。固氮类菌体使用由fixNOQP操纵子编码的细胞色素cbb3型氧化酶；电子通过细胞色素bc1复合体传递到这种高亲和力氧化酶。在自由生活生长期间，电子从细胞色素bc1复合体传递到细胞色素aa3是通过跨膜细胞色素c（CycM）进行的。在一些根瘤菌（如日本慢生根瘤菌）中，还有第二种细胞色素氧化酶，它也需要通过细胞色素bc1复合体进行电子传递。与这些细胞色素c氧化酶同时存在的是在自由生活生长期间表达的喹啉氧化酶。据认为，日本慢生根瘤菌中存在细胞色素bb3喹啉氧化酶；在豌豆根瘤菌、菜豆根瘤菌和茎瘤固氮根瘤菌中，已经鉴定出细胞色素d型氧化酶。光谱数据表明，几种根瘤菌中存在细胞色素o型氧化酶，尽管日本慢生根瘤菌中不存在血红素O，这可能表明归因于细胞色素o的吸收可能是由于细胞色素bb3型氧化酶中的高自旋细胞色素b。在一些根瘤菌中，参与细胞色素c组装的基因突变对生长影响不大，推测是因为细菌利用了不依赖细胞色素c的喹啉氧化酶。在这篇综述中，我们概述了关于电子传递途径不同组分受影响的各种根瘤菌突变体的研究工作，以及这些突变对共生固氮和自由生活生长所产生的影响。
