Fnr is a transcriptional regulator that controls the expression of a variety of genes in response to oxygen limitation in bacteria. Genome sequencing revealed four genes (fnr1, fnr3, fnr5 and fnr7) coding for Fnr proteins in Paenibacillus polymyxa WLY78. Fnr1 and Fnr3 showed more similarity to each other than to Fnr5 and Fnr7. Also, Fnr1 and Fnr3 exhibited high similarity with Bacillus cereus Fnr and Bacillus subtilis Fnr in sequence and structures. Both of the aerobically purified His-tagged Fnr1 and His-tagged Fnr3 in Escherichia coli could bind to the specific DNA promoter. Deletion analysis showed that the four fnr genes, especially fnr1 and fnr3, have significant impacts on growth and nitrogenase activity. Single deletion of fnr1 or fnr3 led to a 50% reduction in nitrogenase activity and double deletion of fnr1 and fnr3 resulted to a 90% reduction in activity. Genome-wide transcription analysis showed that Fnr1 and Fnr3 indirectly activated expression of nif (nitrogen fixation) genes and Fe transport genes under anaerobic conditions. Fnr1 and Fnr3 inhibited expression of the genes involved in the aerobic respiratory chain and activated expression of genes responsible for anaerobic electron acceptor genes.

IMPORTANCE

The members of the nitrogen-fixing Paenibacillus spp. have great potential to be used as a bacterial fertilizer in agriculture. However, the functions of fnr gene(s) in nitrogen fixation and other metabolisms in Paenibacillus spp. are not known. Here, we found that in P. polymyxa WLY78, Fnr1 and Fnr3 were responsible for regulation of numerous genes in response to changes in oxygen levels, but Fnr5 and Fnr7 exhibited little effect. Fnr1 and Fnr3 indirectly or directly regulated many types of important metabolism, such as nitrogen fixation, Fe uptake, respiration and electron transport. This study not only reveals the function of the fnr genes of P. polymyxa WLY78 in nitrogen fixation and other metabolisms, but also will provide insight into the evolution and regulatory mechanisms of fnr in Paenibacillus.

Fnr是一种转录调节因子，可响应细菌中的氧气限制而控制多种基因的表达。基因组测序揭示了编码多粘芽孢杆菌WLY78中Fnr蛋白的四个基因（fnr1，fnr3，fnr5和fnr7）。与Fnr5和Fnr7相比，Fnr1和Fnr3显示出更多的相似性。此外，Fnr1和Fnr3在序列和结构上与蜡状芽孢杆菌Fnr和枯草芽孢杆菌Fnr表现出高度相似性。需氧纯化的大肠杆菌中带有His标签的Fnr1和带有His标签的Fnr3均可与特定的DNA启动子结合。缺失分析表明四个fnr基因，特别是fnr1和fnr3，对生长和固氮酶活性有重要影响。fnr1或fnr3的单缺失导致固氮酶活性降低50％，fnr1和fnr3的双缺失导致活性降低90％。全基因组转录分析表明，Fnr1和Fnr3在厌氧条件下间接激活了nif（固氮）基因和Fe转运基因的表达。Fnr1和Fnr3抑制了好氧呼吸链中涉及的基因的表达，并激活了负责厌氧电子受体基因的基因的表达。

重要性

固氮芽孢杆菌属的成员。在农业上有作为细菌肥料的巨大潜力。但是，fnr基因在Paenibacillus spp的固氮和其他代谢中的功能。不知道。在这里，我们发现在P. polymyxa WLY78中，Fnr1和Fnr3负责调节许多基因以响应氧水平的变化，但是Fnr5和Fnr7的作用很小。Fnr1和Fnr3间接或直接调节许多类型的重要代谢，例如固氮，铁吸收，呼吸和电子传递。这项研究不仅揭示了P. polymyxa的fnr基因的功能WLY78固氮和其他代谢，同时也将提供洞察演变和监管机制FNR在类芽孢杆菌。