Quality control in the bacterial periplasm.

Studies of the mechanisms that Gram-negative bacteria use to sense and respond to stress have led to a greater understanding of protein folding in both cytoplasmic and extracytoplasmic locations. In response to stressful conditions, bacteria induce a variety of stress response systems, examples of which are the sigma(E) and Cpx systems in Escherichia coli. Induction of these stress response systems results in upregulation of several gene targets that have been shown to be important for protein folding under normal conditions. Here we review the identification of stress response systems and their corresponding gene targets in E. coli. In addition, we discuss the apparent redundancy of the folding factors in the periplasm, and we consider the potential importance of the functional overlap that exists.