Toward a digital gene response: RNA G-quadruplexes with fewer quartets fold with higher cooperativity

Changes in RNA conformation can alter gene expression. The guanine quadruplex sequence (GQS) is an RNA motif that folds in the presence of K ⁺ ions. Changes in the conformation of this motif could be especially important in regulating gene expression in plants because intracellular K ⁺ concentrations often increase during drought stress. Little is known about the folding thermodynamics of RNA GQS. We show here that RNA GQS with tracts containing three G's [e.g., (GGGxx) ₄] have a modest dependence on the K ⁺ concentration, folding with no or even negative cooperativity (Hill coefficients ≤1), and are associated with populated folding intermediates. In contrast, GQS with tracts containing just two G's [e.g., (GGxx) ₄] have a steep dependence on the K ⁺ concentration and fold with positive cooperativity (Hill coefficients of 1.7-2.7) without significantly populating intermediate states. We postulate that in plants, the more stable G3 sequences are largely folded even under unstressed conditions, while the less stable G2 sequences fold only at the higher K ⁺ concentrations associated with cellular stress, wherein they respond sharply to changing K ⁺ concentrations. Given the binary nature of their folding, G2 sequences may find application in computation with DNA and in engineering of genetic circuits.