Two RNA recognition motif-containing proteins are plant mitochondrial editing factors

Post-transcriptional C-to-U RNA editing occurs in plant plastid and mitochondrial transcripts. Members of the Arabidopsis RNA-editing factor interacting protein (RIP) family and ORRM1 (Organelle RNA Recognition Motif-containing protein 1) have been recently characterized as essential components of the chloroplast RNA editing apparatus. ORRM1 belongs to a distinct clade of RNA Recognition Motif (RRM)-containing proteins, most of which are predicted to be organelle-targeted. Here we report the identification of two proteins, ORRM2 (organelle RRM protein 2) and ORRM3 (organelle RRM protein 3), as the first members of the ORRM clade to be identified as mitochondrial editing factors. Transient silencing of ORRM2 and ORRM3 resulted in reduced editing efficiency at ∼6% of the mitochondrial C targets. In addition to an RRM domain at the N terminus, ORRM3 carries a glycine-rich domain at the C terminus. The N-terminal RRM domain by itself provides the editing activity of ORRM3. In yeast-two hybrid assays, ORRM3 interacts with RIP1, ORRM2 and with itself. Transient silencing of ORRM2 in the orrm3 mutant further impairs the editing activity at sites controlled by both ORRM2 and ORRM3. Identification of the effect of ORRM2 and ORRM3 on RNA editing reveals a previously undescribed role of RRM-containing proteins as mitochondrial RNA editing factors.