A third envelope stress signal transduction pathway in Escherichia coli

Summary Escherichia coliuses overlapping envelope stress responses to adapt to insults to the bacterial envelope that cause protein misfolding. The σEand 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 σEpathway 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 thespygene as a new Cpx regulon member of unknown function. Interestingly, induction ofspyexpression 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 ofspyin response to envelope stress. BaeS and BaeR do not affect expression of other known Cpx‐regulated genes, however,baeR cpxRdouble 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 inE. coli that induces expression of a distinct set of adaptive genes.