Thermogenetic tools to monitor temperature-dependent gene expression in bacteria.

Free-living bacteria constantly monitor their ambient temperature. Drastic deviations elicit immediate protective responses known as cold shock or heat shock response. Many mammalian pathogens use temperature surveillance systems to recognize the successful invasion of a host by its body temperature, usually 37°C. Translation of temperature-responsive genes can be modulated by RNA thermometers (RNATs). RNATs form complex structures primarily in the 5'-untranslated region of their transcripts. Most RNATs block the ribosome binding site at low temperatures. Translation is induced at increasing temperature by melting of the RNA structure. The analysis of such temperature-dependent RNA elements calls for adequate test systems that function in the appropriate temperature range. Here, we summarize previously established reporter gene systems based on the classical ß-galactosidase LacZ, the heat-stable ß-galactosidase BgaB and the green fluorescent protein GFP. We validate these systems by testing known RNATs and describe the construction and application of an optimized bgaB system. Finally, two novel RNA thermometer candidates from Escherichia coli and Salmonella will be presented.

Multiple layers of control govern expression of the Escherichia coli ibpAB heat-shock operon.

The Escherichia coli ibpAB operon encodes two small heat-shock proteins, the inclusion-body-binding proteins IbpA and IbpB. Here, we report that expression of ibpAB is a complex process involving at least four different layers of control, namely transcriptional control, RNA processing, translation control and protein stability. As a typical member of the heat-shock regulon, transcription of the ibpAB operon is controlled by the alternative sigma factor σ(32) (RpoH). Heat-induced transcription of the bicistronic operon is followed by RNase E-mediated processing events, resulting in monocistronic ibpA and ibpB transcripts and short 3'-terminal ibpB fragments. Translation of ibpA is controlled by an RNA thermometer in its 5' untranslated region, forming a secondary structure that blocks entry of the ribosome at low temperatures. A similar structure upstream of ibpB is functional in vitro but not in vivo, suggesting downregulation of ibpB expression in the presence of IbpA. The recently reported degradation of IbpA and IbpB by the Lon protease and differential regulation of IbpA and IbpB levels in E. coli are discussed.

The Escherichia coli ibpA thermometer is comprised of stable and unstable structural elements.

Translation of many small heat shock genes in alpha- and gamma-proteobacteria is controlled by the ROSE (Repression Of heat Shock gene Expression) element, a thermo-responsive RNA structure in the 5'-untranslated region. ROSE(ibpA) regulates translation of the Escherichia coli ibpA gene coding for an inclusion body-associated protein. We present first structural insights into a full-length ROSE element by examining the temperature-induced conformational changes of ROSE(ibpA) using detailed enzymatic and lead probing experiments between 20 and 50 degrees C. The initial two hairpins are stable at all temperatures tested and might assist in proper folding of the third temperature-responsive stem-loop structure, which restricts access to the Shine-Dalgarno sequence at temperatures below 35 degrees C. Toeprinting (primer extension inhibition) experiments show that binding of the 30S ribosome to ROSE(ibpA) is enhanced at high temperatures. In contrast to other ROSE-like elements, the final hairpin is rather short. Single point mutations result in alternative structures with positive or negative effects on translation efficiency. Our study demonstrates how the combination of stable and unstable modules controls translation efficiency in a complete RNA thermometer.

Genome-wide bioinformatic prediction and experimental evaluation of potential RNA thermometers.

Only recently, the fundamental role of regulatory RNAs in prokaryotes and eukaryotes has been appreciated. We developed a pipeline from bioinformatic prediction to experimental validation of new RNA thermometers. Known RNA thermometers are located in the 5'-untranslated region of certain heat shock or virulence genes and control translation by temperature-dependent base pairing of the ribosome binding site. We established the searchable database RNA-SURIBA (Structures of Untranslated Regions In BActeria). A structure-based search pattern reliably recognizes known RNA thermometers and predicts related structures upstream of annotated genes in complete genome sequences. The known ROSE(1) (Repression Of heat Shock gene Expression) thermometer and several other functional ROSE-like elements were correctly predicted. For further investigation, we chose a new candidate upstream of the phage shock gene D (pspD) in the pspABCDE operon of E. coli. We established a new reporter gene system that measures translational control at heat shock temperatures and we demonstrated that the upstream region of pspD does not confer temperature control to the phage shock gene. However, translational efficiency was modulated by a point mutation stabilizing the predicted hairpin. Testing other candidates by this structure prediction and validation process will lead to new insights into the requirements for biologically active RNA thermometers. The database is available on http://www.ruhr-uni-bochum.de/mikrobiologie/ .