The role of plant-specific PPR proteins in chloroplast RNA editing

RNA editing is a post-transcriptional process that alters specific cytidine residues to uridine in the mitochondrial and chloroplast transcripts of higher plants and occurs frequently. Our analysis of mutants affected in chloroplast NAD(P)H dehydrogenase function has revealed that specific target C sites are recognized by each site-specific factors; the PLS subfamily of pentatricopeptide repeat (PPR) proteins that is specific to land plants. The PLS subfamily of PPR proteins is composed by a tandem array of PPR motifs to bind RNA and additional E and DYW motifs at the C-terminus. Based on the differences in C-terminal motifs, the PLS subfamily is classified into the E and DYW subclasses. To clarify each role of two types of site-specific factors in RNA editing, we focused on the E and DYW motifs and investigated their functions. Based on the results, we propose that the E-subclass PPR proteins recognize a specific RNA sequence motif, binds there, and recruits one or more additional proteins with the enzymatic activity through protein-protein interaction with the E motif. Moreover, our results suggests that the DYW-subclass PPR proteins might behave exactly like the E-subclass editing specificity factors despite the presence of the additional DYW motif.