Fig 4. Increased levels of σ⁷⁰ reduce σ³²-dependent expression.

(A) σ⁷⁰ steady-state levels during DnaKJ depletion in the P_rpoD(Δ11bp) background compared to the unsuppressed DnaKJ depletion background, P_rpoD (WT). Quantifications are shown as the means of independent triplicates. Error bars represent standard deviations. (B) Spot assay of the DnaKJ depletion strain harboring either the empty vector pBVMCS-2 or the pBVMCS-2-P_vanA-rpoD plasmid on PYE agar plates containing glucose. Vanillate induces rpoD expression. (C) Model illustrating how the different suppressor mutations reduce σ³² activity. In the wild type (WT) DnaKJ inactivates σ³² and primes it for degradation by the protease FtsH, thus inhibiting expression from the σ³²-regulon. Depletion of DnaKJ leads to stabilization and activation of σ³², which is now free to associate with the RNAP and to induce σ³²-dependent gene expression. Mutations restoring growth in the absence of DnaKJ lower σ³² activity, either by loss of function amino acid exchanges in the heat shock sigma factor itself, by inducing its degradation through the protease HslUV, by lowering the affinity of RNAP for σ³² or by increasing the levels of σ⁷⁰ that outcompetes σ³² for binding to the RNAP core.