Role of Two RpoH Transcription Factors Required for the Rhizobium-legume Symbiosis

复制现有的调节系统并适应新的用途是在生物体中产生复杂性的一种方法。在α -变形细菌中，有一部分物种含有两个或多个rpoH基因拷贝，该基因编码RNA聚合酶的另一个sigma因子亚基，该亚基在某些启动子处启动转录。RpoH首先在大肠杆菌中被发现是一种热休克sigma因子，它指导编码热休克蛋白（HSPs）的基因转录，如伴侣蛋白和蛋白酶，在热休克和正常生长条件下都在蛋白质折叠和质量控制中发挥作用。为什么一些α -变形菌维持多个rpoH基因？转录因子是单独控制经典热休克蛋白还是被招募来调节其他途径？每个根瘤菌的基因组测序都包含多个rpoH基因。这些在农业上很重要的细菌将氮固定成供寄主植物使用的形式，以换取营养。在墨氏中华根瘤菌中有两个rpoH基因，它们是结瘤和固氮所必需的。本研究的具体目的是：(i)通过转录谱分析策略鉴定受RpoH1和RpoH2控制的基因；（ii）通过确定自由生活和共生条件下突变体的表型，表征所有个体靶基因对RpoH1和RpoH2表型的贡献。该项目的更广泛影响包括将根瘤菌-豆科植物共生应用于面向高中学生和教师的推广项目，培训本科生和研究生，以及更新一门遗传学课程。

Duplication of an existing regulatory system and adaptation to a new use is one approach for generating complexity within an organism. Among the alpha-proteobacteria, a subset of species contains two or more copies of the rpoH gene, which encodes an alternative sigma factor subunit of RNA polymerase that initiates transcription at certain promoters. RpoH was first identified in E. coli as a heat shock sigma factor that directs transcription of genes encoding heat shock proteins (HSPs), such as chaperones and proteases, which play roles in protein folding and quality control under both heat shock and normal growth conditions. Why do some alpha-proteobacteria maintain multiple rpoH genes? Are the transcription factors solely controlling the classic HSPs or have they been recruited to regulate other pathways? Every root-nodulating Rhizobium species whose genome has been sequenced contains multiple rpoH genes. These agriculturally important bacteria fix nitrogen into a usable form for host plants in exchange for nutrients. In Sinorhizobium meliloti there are two rpoH genes, which are required for nodulation and nitrogen fixation. The specific aims of this research are (i) to identify the genes controlled by RpoH1 and RpoH2 by employing transcriptional profiling strategies and (ii) to characterize the contribution of all the individual target genes to the rpoH1 and rpoH2 phenotypes by determining the phenotypes of mutants under free-living and symbiotic conditions. The broader impacts of the project include using the Rhizobium-legume symbiosis in outreach programs for high school students and teachers, training undergraduate and graduate students, and updating a genetics course.