| Abstract |
When Escherichia coli cells are transferred from 30 degrees C to 42 degrees C, transcription from specific promoters recognized by RNA polymerase containing sigma 32 (the rpoH gene product) is transiently activated, resulting in induction of heat shock proteins. Transcription from heat shock promoters is activated by an increased cellular concentration of sigma 32 due to enhanced synthesis and stabilization. We have constructed and examined the expression of mutant derivatives (deletions and base substitutions) of rpoH-lacZ gene fusion. Synthesis of a sigma 32-beta-galactosidase fusion protein was found to be regulated at the translational level involving two distinct 5'-proximal rpoH coding regions. A small region immediately downstream of the initiation codon is required for potentially high-level expression, whereas a much larger internal region is required for thermal regulation--namely, repression at low temperature or nonstress conditions. The two mRNA regions act as positive and negative cis elements, respectively, in controlling rpoH translation. We propose that an interplay between these RNA regions involving secondary structure formation is important in regulating translation initiation and that transient disruption of secondary structure represents a primary step of the heat shock response.
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