Crystal structure of the chloroplast RNA editing factor MORF2.

RNA editing is a post-transcription process that alters the genetic information on RNA molecules. In plastids and mitochondria of flowering plants, the multiple organellar RNA editing factors (MORFs) interact with the PLS-type pentatricopeptide repeat (PPR) proteins and participate in RNA editing of cytidine-to-uridine conversion. The PPR proteins recognize cytidine targets around the editing sites, and the MORF proteins modulate the RNA-binding activity of the PPR proteins. Here, we report the structure of the Arabidopsis thaliana chloroplast MORF2 at 2.4 Å resolution. The structure, adopting typical MORF-box fold as observed in mitochondrial MORF1 and chloroplast MORF9, reveals an MORF1-like dimerization mode. The difference between the two dimerization modes can be attributed to F157 (corresponding F162 in MORF1 and W160 in MORF9), which causes a 60° shift upon dimerization. This observation, together with the PPR-MORF2 model, suggests a dimer-to-monomer transition during RNA editosome formation.