A mRNA-based thermosensor controls expression of rhizobial heat shock genes

Expression of several heat shock operons, mainly coding for small heat shoc k proteins, is under the control of ROSE (repression of heat shock gene exp ression) in various rhizobial species. This negatively cis-acting element c onfers temperature control by preventing expression at physiological temper atures. We provide evidence that ROSE-mediated regulation occurs at the pos t-transcriptional level. A detailed mutational analysis of ROSE1-hspA trans lationally fused to lacZ revealed that its highly conserved 3'-half is requ ired for repression at normal temperatures (30 degreesC). The mRNA in this region is predicted to form an extended secondary structure that looks very similar in all 15 known ROSE elements. Nucleotides involved in base pairin g are strongly conserved, whereas nucleotides in loop regions are more dive rgent. Base substitutions leading to derepression of the lacZ fusion at 30 degreesC exclusively resided in potential stem structures. Optimised base p airing by elimination of a bulged residue and by introduction of complement ary nucleotides in internal loops resulted in ROSE elements that were tight ly repressed not only at normal but also at heat shock temperatures. We pro pose a model in which the temperature-regulated secondary structure of ROSE mRNA influences heat shock gene expression by controlling ribosome access to the ribosome-binding site.