RESUMEN
Magnesium, the most abundant divalent cation in cells, catalyzes RNA cleavage but also promotes RNA folding. Because folding can protect RNA from cleavage, we predicted a 'Goldilocks landscape', with local maximum in RNA lifetime at Mg2+ concentrations required for folding. Here, we use simulation and experiment to discover an innate and sophisticated mechanism of control of RNA lifetime. By simulation we characterized RNA Goldilocks landscapes and their dependence on cleavage and folding parameters. Experiments with yeast tRNAPhe and the Tetrahymena ribozyme P4-P6 domain show that structured RNAs can inhabit Goldilocks peaks. The Goldilocks peaks are tunable by differences in folded and unfolded cleavage rate constants, Mg2+ binding cooperativity, and Mg2+ affinity. Different folding and cleavage parameters produce Goldilocks landscapes with a variety of features. Goldilocks behavior allows ultrafine control of RNA chemical lifetime, whereas non-folding RNAs do not display Goldilocks peaks of protection. In sum, the effects of Mg2+ on RNA persistence are expected to be pleomorphic, both protecting and degrading RNA. In evolutionary context, Goldilocks behavior may have been a selectable trait of RNA in an early Earth environment containing Mg2+ and other metals.