Department of Microbiology and Immunology, University of Otago, Dunedin, New Zealand.
PLoS One. 2013;8(1):e53762. doi: 10.1371/journal.pone.0053762. Epub 2013 Jan 7.
Mesorhizobium loti is the microsymbiont of Lotus species, including the model legume L. japonicus. M. loti differs from other rhizobia in that it contains two copies of the key nitrogen fixation regulatory gene nifA, nifA1 and nifA2, both of which are located on the symbiosis island ICEMlSym(R7A). M. loti R7A also contains two rpoN genes, rpoN1 located on the chromosome outside of ICEMlSym(R7A) and rpoN2 that is located on ICEMlSym(R7A). The aims of the current work were to establish how nifA expression was activated in M. loti and to characterise the NifA-RpoN regulon. The nifA2 and rpoN2 genes were essential for nitrogen fixation whereas nifA1 and rpoN1 were dispensable. Expression of nifA2 was activated, possibly in response to an inositol derivative, by a novel regulator of the LacI/GalR family encoded by the fixV gene located upstream of nifA2. Other than the well-characterized nif/fix genes, most NifA2-regulated genes were not required for nitrogen fixation although they were strongly expressed in nodules. The NifA-regulated nifZ and fixU genes, along with nifQ which was not NifA-regulated, were required in M. loti for a fully effective symbiosis although they are not present in some other rhizobia. The NifA-regulated gene msi158 that encodes a porin was also required for a fully effective symbiosis. Several metabolic genes that lacked NifA-regulated promoters were strongly expressed in nodules in a NifA2-dependent manner but again mutants did not have an overt symbiotic phenotype. In summary, many genes encoded on ICEMlSym(R7A) were strongly expressed in nodules but not free-living rhizobia, but were not essential for symbiotic nitrogen fixation. It seems likely that some of these genes have functional homologues elsewhere in the genome and that bacteroid metabolism may be sufficiently plastic to adapt to loss of certain enzymatic functions.
根瘤菌属中慢生根瘤菌是豆科植物包括模式豆科植物百脉根属的共生菌。中慢生根瘤菌不同于其他根瘤菌的特点是它包含两个关键固氮调控基因 nifA 的拷贝，nifA1 和 nifA2，这两个基因都位于共生 island ICEMlSym(R7A)上。中慢生根瘤菌 R7A 还包含两个 rpoN 基因，rpoN1 位于染色体上 ICEMlSym(R7A)之外，rpoN2 位于 ICEMlSym(R7A)上。本研究的目的是确定中慢生根瘤菌中 nifA 的表达是如何被激活的，并对 NifA-RpoN 调控子进行了表征。nifA2 和 rpoN2 基因对固氮是必需的，而 nifA1 和 rpoN1 是可有可无的。nifA2 的表达是通过位于 nifA2 上游的 fixV 基因编码的新型 LacI/GalR 家族调控因子被激活的，可能是对肌醇衍生物的响应。除了特征明确的 nif/fix 基因外，大多数受 NifA2 调控的基因虽然在根瘤中强烈表达，但对固氮不是必需的。在中慢生根瘤菌中，NifA 调控的 nifZ 和 fixU 基因以及不受 NifA 调控的 nifQ 基因虽然在其他一些根瘤菌中不存在，但对于有效的共生作用是必需的。NifA 调控的 msi158 基因编码一种孔蛋白，对于有效的共生作用也是必需的。几个缺乏 NifA 调控启动子的代谢基因以 NifA2 依赖的方式在根瘤中强烈表达，但突变体没有明显的共生表型。总之，ICEMlSym(R7A)上编码的许多基因在根瘤中强烈表达，但在自由生活的根瘤菌中不表达，但对共生固氮不是必需的。似乎这些基因中的一些在基因组的其他地方具有功能同源物，并且类细菌代谢可能具有足够的可塑性来适应某些酶功能的丧失。
