RNA editing in plant organelles. Why make it easy?

Gene expression in plant organelles involves a number of distinct co- or posttranscriptional nucleic acid modifications: 5' and 3' RNA processing, cis- and trans-splicing, RNA stability, and RNA editing. All contribute to the steady-state RNA levels available for the translation of the reduced but essential organellar genetic information. Different from other maturation processes, RNA editing at the transcript level modifies the information encoded by organellar genes and is an essential step for the production of functional proteins. Editing changes are extensive in mitochondria from flowering plants with more than 400 cytidine-to-uridine changes that involve most transcripts, while in chloroplasts they are limited to some RNAs. An additional U-to-C RNA editing reaction is observed with the C-to-U transitions in fern and moss organelles. While RNA editing targets mostly concern coding regions, some events occur in untranslated regions. Whereas RNA editing is genetically and biochemically distinct from other RNA modification activities, evidence is growing for a tight connection between the different processing events. Although the understanding of this astonishing mechanism has increased since its discovery in 1989, some important questions remain unanswered. In this review we discuss the current knowledge on the different aspects of C-to-U, and to a lesser extent U-to-C, and look at RNA editing in plants with a particular emphasis on recent developments involving the role of pentatricopeptide repeat (PPR) proteins in this process.