Sense from nonsense: how the genetic information of chloroplasts is altered by RNA editing

Plastid transcripts can be subject to an RNA processing mechanism changing the identity of individual nucleotides and thus altering the information content of the mRNA. This processing step was termed RNA editing and adds a novel mechanism to the multitude of RNA maturation events required before mRNAs can serve as faithful templates in plastid protein biosynthesis. RNA editing in chloroplasts proceeds by the conversion of individual cytidine residues to uridine and, in some bryophytes, also by the reverse event, uridine-to-cytidine transitions. The discovery of RNA editing in chloroplasts has provided researchers with a wealth of molecular and evolutionary puzzles, many of which are not yet solved. However, recent work employing chloroplast transformation technologies has shed some light on the molecular mechanisms by which RNA editing sites are recognized with extraordinarily high precision. Also, extensive phylogenetic studies have provided intriguing insights in the evolutionary dynamics with which editing sites may come and go. This review summarizes the state-of-the-art in the field of chloroplast RNA editing, discusses mechanistic and evolutionary aspects of editing and points out some of the important open questions surrounding this enigmatic RNA processing step.