Accessory proteins increase the efficiency of RNA editing by Arabidopsis chloroplast editosomes

RNA editing modifies cytidines to uridines in plant organelle transcripts so that their sequence differs from the one predicted from the genomic DNA. This corrective mechanism allows the production of functional proteins that would otherwise be hampered by deleterious mutations. This process conserved across most land plants involves a family of RNA-binding proteins that has significantly expanded, the pentatricopeptide repeat (PPR) containing proteins. In moss, a non-vascular plant, a single PPR protein is responsible for both binding and deaminating the RNA target. In angiosperms PPR proteins are found in editosomes associated with accessory proteins. The exact function of these accessory proteins has been unclear. Bacterial co-expression of an angiosperm synthetic factor and different accessory proteins, RIP2, RIP9, ORRM1 demonstrates their essential role in editing of an RNA target. The presence of ORRM1 and RIP2 or ORRM1 and RIP9 in bacteria with the PPR factor results in a target editing extent of 80%, which is similar to what is observed in planta . Accessory proteins increase the affinity of the PPR factor for the target RNA, likely the explanation of their role in improving editing efficiency.