Escherichia coli YaeJ protein mediates a novel ribosome-rescue pathway distinct from SsrA- and ArfA-mediated pathways

CtsR, the global heat shock repressor in low GC, Gram+ bacteria, regulates a crucial subset of genes involved in protein quality control. CtsR de-repression occurs not only during heat stress but also during a variety of other environmental stresses, most notably thiol-specific oxidative stress. Here we report that McsA acts as a molecular redox switch that regulates CtsR de-repression via the activation of McsB. Once critical thiols of McsA become oxidized, the strong interaction between McsA and McsB is interrupted and free McsB is no longer inhibited by McsA, resulting in the inactivation of CtsR. This mechanism differs significantly from inactivation of CtsR during heat stress demonstrating a dual activity control of CtsR. Moreover, we show that in those low GC, Gram+ bacteria, which lack the McsA/McsB complex, the Zn finger protein ClpE is able to sense and respond to oxidative stress, also resulting in CtsR inactivation.