RNA thermometers are a regulatory structure found in some RNA transcripts, controlling the ability of the ribosome to bind based on the temperature of the environment. A stem-loop in the RNA secondary structure melts over a range of temperatures, with the specific temperature being dependent on the G-C content of the loop structure.
As the thermometer structure melts over a range of temperatures, expression gradually changes. This is not an ‘all or nothing’ response.
This regulation is used with heat-shock proteins and virulence factors (to detect when the pathogen is inside a host).