Metals induce transient folding and activation of the Twister ribozyme

Twister is a small ribozyme present in almost all kingdoms of life that rapidly self-cleaves in variety of divalent metal ions. We used activity assays, bulk FRET and single-molecule FRET (smFRET) to understand how different metal ions promote folding and self-cleavage of the Oryza sativa Twister ribozyme. Although most ribozymes require additional Mg(2+) for catalysis, Twister inverts this expectation, requiring 20–30 times less Mg(2+) to self-cleave than to fold. Transition metals such as Co(2+), Ni(2+) and Zn(2+) activate Twister more efficiently than Mg(2+) ions. Although Twister is fully active in ≤ 0.5 mM MgCl(2), smFRET experiments showed that the ribozyme visits the folded state infrequently under these conditions. Comparison of folding and self-cleavage rates indicates that most folding events lead to catalysis, which correlates with metal bond strength. Thus, the robust activity of Twister reports on transient metal ion binding under physiological conditions.