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

Induction of heat shock proteins in Escherichia coli is primarily caused by increased cellular levels of the heat shock sigma-factor sigma(32) encoded by the rpoH gene. Increased sigma(32) levels result from both enhanced syn thesis and stabilization. Previous work indicated that sigma(32) synthesis is induced at the translational level and is mediated by the mRNA secondary structure formed within the 5'-coding sequence of rgoH, including the tran slation initiation region. To understand the mechanism of heat induction of sigma(32) synthesis further, we analyzed expression of rpoH-lacZ gene fusi ons with altered stability of mRNA structure before and after heat shock. A clear correlation was found between the stability and expression or the ex tent of heat induction. Temperature-melting profiles of mRNAs with or witho ut mutations correlated well with the expression patterns of fusion genes c arrying the corresponding mutations in vivo. Furthermore, temperature depen dence of mRNA-30S ribosome-tRNA(f)(Met) complex formation with wild-type or mutant mRNAs in vitro agreed well with that of the expression of gene fusi ons in vivo. Our results support a novel mechanism in which partial melting of mRNA secondary structure at high temperature enhances ribosome entry an d translational initiation without involvement of other cellular components , that is, intrinsic mRNA stability controls synthesis of a transcriptional regulator.