Transition metal ions (e.g., manganese, Mn) serve as redox centers in enzymes, especially those that help cells fight oxidative stress. Excess Mn is toxic to cells because it replaces similarly sized and charged iron in iron-dependent enzymes, inactivating them due to the different redox potential. Bacterial cells thus tightly regulate intracellular Mn content by tuning expression of Mn importers and exporters. One such regulation mechanism is via riboswitches, which are 5’-untranslated regions of mRNA that promote or inhibit expression of the downstream gene by changing their folding upon binding a ligand (e.g., Mn ion). A riboswitch is comprised of an aptamer domain, which detects the presence of a specific ligand, overlapping with an expression platform, whose folding ultimately determines the transcriptional and/or translational fate of the mRNA. Here, I will describe my lab’s collaborative work on two Mn-sensing riboswitches in E. coli , both of which control expression of Mn exporters. With single-molecule FRET, in vivo gene reporters, and chemical probing of RNA structure, we observe that even though the two riboswitches belong to the same large family of Mn-responsive riboswitches, they have distinct mechanisms of Mn sensing, starkly different sensitivity to Mn concentration, and divergent pH-dependence of Mn binding and subsequent gene expression outcome. I will share our findings into the molecular basis behind this divergent behavior of the two RNAs, providing general insights into the role of RNA pK a in regulating its structure and function.
Palmer et al. (Sun,) studied this question.