Crystal structure of an RNA polymerase ribozyme in complex with an antibody fragment

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Abstract
All models of the RNA world era invoke the presence of ribozymes that can catalyse RNA polymerization. The class I ligase ribozyme selected in vitro 15 years ago from a pool of random RNA sequences catalyses formation of a 3′,5′-phosphodiester linkage analogous to a single step of RNA polymerization. Recently, the three-dimensional structure of the ligase was solved in complex with U1A RNA-binding protein and independently in complex with an antibody fragment. The RNA adopts a tripod arrangement and appears to use a two-metal ion mechanism similar to protein polymerases. Here, we discuss structural implications for engineering a true polymerase ribozyme and describe the use of the antibody framework both as a portable chaperone for crystallization of other RNAs and as a platform for exploring steps in evolution from the RNA world to the RNA–protein world.