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.
