RseB binding to the periplasmic domain of RseA modulates the RseA :sigma(E) interaction in the cytoplasm and the availability of sigma(E)center dot RNA polymerase

The Escherichia coli sigma (E) regulon has evolved to sense the presence of misfolded proteins in the bacterial envelope. Expression of periplasmic ch aperones and folding catalysts is under the control of sigma (E) RNA polyme rase. The N-terminal domain of RseA sequesters sigma (E) in the cytoplasmic membrane, preventing its association with core RNA polymerase, The C-termi nal domain of RseA interacts with RseB, a periplasmic protein. The relative concentration of sigma (E):RseA:RseB is 2:5:1 and this ratio remains unalt ered upon heat shock induction of the sigma (E) regulon. Purification from crude cellular extracts yields cytoplasmic, soluble sigma (E) RNA polymeras e as web as membrane sequestered sigma (E) RseA and sigma (E).RseA.RseB. Rs eB binding to the C-terminal domain of RseA increases the affinity of RseA for sigma (E) by 2- to 3-fold (K-d, 50-100 nM). RseB binds also to the misf olded aggregates of MalE31, a variant of maltose binding protein that forms inclusion bodies in the periplasm. We discuss a model whereby the RseB-Rse A interaction represents a measure for misfolded polypeptides in the bacter ial envelope, modulating the assembly of sigma (E) RNA polymerase and the c ellular heat shock response.