During infection, σE of S. Typhimurium is required for survival and proliferation in epithelial and macrophage cell lines, and in the presence of antimicrobial peptides [6, 28, 29]. In Pseudomonas aeruginosa, the σE homologue, AlgU, controls LY2606368 ic50 the expression of the exopolysaccharide alginate and conversion to mucoidy. AlgU is constitutively activated in many clinical isolates from cystic fibrosis patients [30, 31]. In addition, σE is required for the viability of some bacterial species, but not others. The gene CYT387 concentration encoding σE is essential in E. coli and Yersinia enterocolitica,

but is dispensable in the closely related species S. Typhimurium [6, 32, 33]. These observations suggest that the functions of σE orthologs have been adapted to combat the challenges each organism faces in its particular environmental niche. By exploring the role of σE in diverse bacterial species, we can learn which aspects of this widespread regulatory pathway are universally conserved and which have diverged over the course

of evolution. Here we show that the B. bronchiseptica σE ortholog, encoded by the gene sigE (BB3752), is an active sigma factor that mediates a cell envelope stress response. This is the first demonstration of an envelope stress-sensing system in see more Bordetella species. Using a murine infection model, we demonstrate that SigE plays an important role during lethal infection in mice lacking adaptive immunity, but not in respiratory tract colonization. This finding has important implications for human disease, given the observation that B. bronchiseptica can cause serious systemic infections in immunocompromised humans [11, 14]. This study

suggests that SigE is a critical factor in this process, in addition to the BvgAS master virulence regulatory system. Results sigE encodes an active sigma factor The sigE gene of B. bronchiseptica shares pheromone a number of conserved residues with other members of the RpoE-like sigma factors, including those in the DNA-binding regions (Figure 1A) [24]. To determine if sigE encodes an active sigma factor, we asked whether it could direct transcription from the σE-dependent rpoHP3 promoter in E. coli. This promoter shares a high degree of similarity with a consensus promoter proposed for the RpoE-like sigma factors that was determined from both experimental data and predicted promoter sequences (Figure 1C) [24, 27]. The sigE gene from B. bronchiseptica strain RB50 was cloned into the pTrc99a expression plasmid and transformed into a derivative of E. coli MG1655 that carries an rpoHP3::lacZ reporter gene fusion integrated on the chromosome [34]. When sigE expression was induced, LacZ activity increased, indicating that SigE can initiate transcription from this promoter (Figure 1B). Furthermore, we found that the gene encoding σE, rpoE, which is essential for viability in E. coli, could be deleted when sigE was overexpressed (data not shown, see Materials and Methods). Figure 1 B. bronchiseptica SigE is a functional sigma factor.