What makes a 5’UTR an RNA thermometer?

Temperature is among the most important parameters that free-living microbes monitor constantly. The microbial physiology needs to be re-adjusted to sudden temperature changes by appropriate heat or cold shock responses. Pathogenic microorganisms often respond to a temperature around 37 °C by inducing virulence gene expression [1, 2]. RNA is well-suited as posttranscriptional intracellular thermosensor. Temperature-responsive RNAs are often referred to as RNA thermometers [3, 4]. All RNA thermometers known thus far are translational control elements and most of them are located in the 5'-UTR of bacterial heat shock and virulence genes. The majority function by a conceptually simple mechanism: At low temperatures, the SD sequence is trapped in a hairpin structure. Increasing temperature destabilizes the structure such that the ribosome binding site becomes accessible, allowing translation to be initiated. In order to understand common principles, we compared potential temperature-sensitive 5'UTRs from a couple of organisms experimentally. Despite a predicted thermometer-like structure, some of these candidates did not respond as expected. Some features that distinguish functional RNA thermometers from other 5'-UTRs will be discussed.