A third envelope stress signal transduction pathway in Escherichia coli

Escherichia coli uses overlapping envelope stress responses to adapt to insults to the bacterial envelope that cause protein misfolding. The sigma(E) and Cpx envelope stress responses are activated by both common and distinct envelope stresses and respond by increasing the expression of the periplasmic protease DegP as well as target genes unique to each response. The sigma(E) pathway is involved in outer membrane protein (OMP) folding quality control whereas the Cpx pathway plays an important role in the assembly of at least one pilus. Previously, we identified the spy gene as a new Cpx regulon member of unknown function. Interestingly, induction of spy expression by severe envelope stresses such as spheroplasting is only partially dependent on an intact Cpx signalling pathway, unlike other Cpx-regulated genes. Here we show that the BaeS sensor kinase and BaeR response regulator also control expression of spy in response to envelope stress. BaeS and BaeR do not affect expression of other known Cpx-regulated genes, however, baeR cpxR double mutants show increased sensitivity to envelope stresses relative to either single mutant alone. We propose that the Bae signal transduction pathway controls a third envelope stress response in E. coli that induces expression of a distinct set of adaptive genes.