Translational induction of heat shock transcription factor sigma 32: evidence for a built-in RNA thermosensor

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Abstract
Induction of heat shock proteins in Escherichia coli is primarily caused by increased cellular levels of the heat shock ς-factor ς32 encoded by the rpoH gene. Increased ς32 levels result from both enhanced synthesis and stabilization. Previous work indicated that ς32 synthesis is induced at the translational level and is mediated by the mRNA secondary structure formed within the 5′-coding sequence ofrpoH, including the translation initiation region. To understand the mechanism of heat induction of ς32 synthesis further, we analyzed expression of rpoH–lacZ gene fusions with altered stability of mRNA structure before and after heat shock. A clear correlation was found between the stability and expression or the extent of heat induction. Temperature-melting profiles of mRNAs with or without mutations correlated well with the expression patterns of fusion genes carrying the corresponding mutations in vivo. Furthermore, temperature dependence of mRNA–30S ribosome–tRNAf Met complex formation with wild-type or mutant mRNAs in vitro agreed well with that of the expression of gene fusions in vivo. Our results support a novel mechanism in which partial melting of mRNA secondary structure at high temperature enhances ribosome entry and translational initiation without involvement of other cellular components, that is, intrinsic mRNA stability controls synthesis of a transcriptional regulator.