Organelle RNA recognition motif-containing (ORRM) proteins are plastid and mitochondrial editing factors in Arabidopsis

Post-transcriptional C-to-U RNA editing occurs at specific sites in plastid and plant mitochondrial transcripts. Members of the Arabidopsis pentatricopeptide repeat (PPR) motif-containing protein family and RNA-editing factor Interacting Protein (RIP, also known as MORF) family have been characterized as essential components of the RNA editing apparatus. Recent studies reveal that several organelle-targeted RNA recognition motif (RRM)-containing proteins are involved in either plastid or mitochondrial RNA editing. ORRM1 (Organelle RRM protein 1) is essential for plastid editing, whereas ORRM2, ORRM3 and ORRM4 are involved in mitochondrial RNA editing. The RRM domain of ORRM1, ORRM3 and ORRM4 is required for editing activity, whereas the auxiliary RIP and Glycine-Rich (GR) domains mediate the ORRM proteins' interactions with other editing factors. The identification of the ORRM proteins as RNA editing factors further expands our knowledge of the composition of the editosome.