The bacterial envelope stress response, which is responsible for sensing stress signals in the envelope and for turning on the sigma(E)-dependent transcription, is modulated by the binding of RseB to RseA. In this study, the solution structures of RseA and its complex with RseB were analyzed using circular dichroism and small-angle X-ray scattering. The periplasmic domain of RseA is unstructured and flexible when it is not bound to RseB. However, upon the formation of the stable complex with RseB, RseA induces conformational changes in RseB and, at the same time, RseA becomes more structured. Furthermore, it appears that some other undefined region of RseA, as well as the previously identified minimum region (amino acid 169-186), is also involved in RseB binding. It is thought that these conformational changes are relevant to the proteolytic cleavage of RseA and the modulation of envelope stress response.