Transcriptional analysis of the rpoE gene encoding extracytoplasmic stress response sigma factor sigmaE in Salmonella enterica serovar Typhimurium.

The rpoE gene of Salmonella enterica serovar Typhimurium (S. Typhimurium), which encodes the extracytoplasmic stress response sigma factor sigmaE, is critically important for the virulence of S. Typhimurium. We analysed expression of rpoE by wild-type and mutant bacteria grown in different conditions by S1-nuclease mapping using RNA, and using in vivo reporter gene fusions. Three promoters, rpoEp1, rpoEp2 and rpoEp3, were located upstream of the S. Typhimurium rpoE gene. The promoters were differentially expressed during growth and under several stress conditions including cold shock. Expression from the rpoEp3 promoter was absent in an S. Typhimurium rpoE mutant, demonstrating its dependence upon sigmaE. The level of mRNA corresponding to rpoEp3 was also higher in a cpxR mutant, indicating a negative regulation of the promoter by the Cpx system. Using this rpoE-dependent promoter, we optimised a two-plasmid system for identification of promoters recognised by S. Typhimurium sigmaE. The rpoEp3 promoter was active in the Escherichia coli two-plasmid system and has an identical transcription start point as in S. Typhimurium but only after induction of S. Typhimurium rpoE expression.