FourU thermometer

Hairpin II appears to be a dynamic feature of FourU's secondary structure. It undergoes a conformational shift when exposed to temperatures above 45 °C, becoming increasingly unpaired as temperature rises. Hairpin I, in contrast, remains stably base-paired in temperatures as high as 50 °C, which implies the structural shift of hairpin II from closed to open may have an important role in heat shock response. A later study used mutant analysis and calculations of enthalpy and entropy to support a cooperative zipper-type unfolding mechanism of FourU hairpin II in response to temperature increase. ==Sigma factor cooperation==

Like other RNA thermometers, FourU is not solely responsible for temperature-dependent expression of its adjacent gene. Instead, it operates in conjunction with a sigma factor (σ32) which is known to also regulate a number of other genes. Sigma factor-RNA thermometer combinations have been found to regulate other heat-shock genes (such as ibpA in E. coli) which has led to speculation of undiscovered RNA thermometers operating alongside sigma factor modules to regulate other related genes as an additional level of control. Further speculation suggests the simpler RNA thermometer method of gene regulation may have evolved prior to the more complex sigma factor transcription control. ==agsA function==

The agsA gene, which is regulated by FourU thermometers, was first discovered in Salmonella enterica. == See also ==