Regulon and promoter analysis of theE. coliheat-shock factor, σ32, reveals a multifaceted cellular response to heat stress

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Abstract
The heat-shock response (HSR), a universal cellular response to heat, is crucial for cellular adaptation. InEscherichia coli, the HSR is mediated by the alternative σ factor, σ32. To determine its role, we used genome-wide expression analysis and promoter validation to identify genes directly regulated by σ32and screened ORF overexpression libraries to identify σ32inducers. We triple the number of genes validated to be transcribed by σ32and provide new insights into the cellular role of this response. Our work indicates that the response is propagated as the regulon encodes numerous global transcriptional regulators, reveals that σ70holoenzyme initiates from 12% of σ32promoters, which has important implications for global transcriptional wiring, and identifies a new role for the response in protein homeostasis, that of protecting complex proteins. Finally, this study suggests that the response protects the cell membrane and responds to its status: Fully 25% of σ32regulon members reside in the membrane and alter its functionality; moreover, a disproportionate fraction of overexpressed proteins that induce the response are membrane localized. The intimate connection of the response to the membrane rationalizes why a major regulator of the response resides in that cellular compartment.