Thermodynamics of the fourU RNA thermal switch derived from molecular dynamics simulations and spectroscopic techniques.

Bacterial 5' untranslated regions of mRNA, termed thermal switches or thermometers, change their structure in response to temperature change. This structural change provides for the regulation of gene expression. One of such thermal switches, called fourU, is present in the Salmonella species. Mutations of fourU were found to abrogate its regulatory properties. We investigated the thermodynamics of the fourU fragment responsible for its structural changes. All-atom molecular dynamics simulations at various temperatures and spectroscopic experiments in solution were performed for the wild-type fourU and its mutants. We found that the U11C and A8C mutations stabilize the fourU structure in comparison to the wild-type fourU, and the double-point G14A/C25U mutant has the most destabilizing effect on the fourU hairpin 2 responsible for temperature sensing. The G14A/C25U mutant is also the easiest to strand-invade by a complementary oligonucleotide as indicated by fluorescence spectroscopy experiments.