Two organelle RNA recognition motif proteins affect distinct sets of RNA editing sites in the plastid.

Plastid and mitochondrial RNAs in vascular plants are subjected to cytidine-to-uridine editing. The model plant species (Arabidopsis) has two nuclear-encoded plastid-targeted organelle RNA recognition motif (ORRM) proteins: ORRM1 and ORRM6. In the mutant, 21 plastid RNA editing sites were affected but none are essential to photosynthesis. In the mutants, two plastid RNA editing sites were affected: -C77 and -C794. Because encodes the β subunit of cytochrome in photosystem II, which is essential to photosynthesis, the mutants were much smaller than the wild type. In addition, the mutants had pale green leaves and reduced photosynthetic efficiency. To investigate the functional relationship between ORRM1 and ORRM6, we generated double homozygous mutants. Morphological and physiological analyses showed that the double mutants had a smaller plant size, reduced chlorophyll contents, and decreased photosynthetic efficiency, similar to the single mutants. Although the double mutants adopted the phenotype of the single mutants, the total number of plastid RNA editing sites affected in the double mutants was the sum of the sites affected in the and single mutants. These data suggest that ORRM1 and ORRM6 are in charge of distinct sets of plastid RNA editing sites and that simultaneous mutations in and genes do not cause additional reduction in editing extent at other plastid RNA editing sites.