A covalent crosslink converts the hammerhead ribozyme from a ribonuclease to an RNA ligase.

The hammerhead ribozyme is a more efficient ribonuclease than an RNA ligase. Under typical reaction conditions, the rate of RNA chain cleavage is approximately 100-fold faster than the rate of the reverse ligation reaction such that virtually all of the hammerhead is in its cleaved form at equilibrium. Here we show that the introduction of a crosslink away from the catalytic core of the hammerhead has little effect on the cleavage rate but dramatically increases the ligation rate, thereby making the hammerhead an efficient RNA ligase. This experiment emphasizes the role of molecular flexibility in defining the properties of a macromolecular catalyst and suggests why other small ribozymes are more efficient ligases than ribonucleases.