H-091. The Regulons of Two RpoH Sigma Factors in Sinorhizobium meliloti
Among the α-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. Although identified as part of the heat shock response, it is clear that RpoH and HSPs also play crucial roles under normal growth conditions. Why do some α-proteobacteria maintain multiple rpoH genes? We hypothesize that they have been recruited to regulate other pathways. Every sequenced root-nodulating Rhizobium species contains multiple rpoH genes. In Sinorhizobium meliloti there are two rpoH genes: rpoH1 mutants are unable to grow at high temperature or symbiotically fix nitrogen, rpoH2 mutants have no phenotype, and rpoH1 rpoH2 double mutants are unable to form nodules. Although the key target of RpoH in E. coli is the chaperone-encoding groESL operon, we have previously demonstrated that the requirement for RpoH during symbiosis is not solely due to a requirement for GroEL/GroES. We have performed microarray analysis to determine the regulons of RpoH1 and RpoH2 in S. meliloti, by comparing gene expression in wild-type cells and rpoH mutant cells during the stationary phase of growth. Analysis of our results suggests that many targets of RpoH1 have not been identified in these arrays. Therefore, we are currently doing microarray analysis using RNA from heat shocked cultures and an inducible system. Our preliminary results suggest that the RpoH1 and RpoH2 regulons are partially overlapping with each other, and target genes include ones that are not part of the E. coli RpoH regulon.