In the course of evolution as endocytobionts, plastids acquired a gene expression system that is by far more complex than the one of their free living cyanobacterial relatives. After transcription, a plastid RNA undergoes a multitude of different processing steps before maturity. Among these processes, RNA editing is the least understood. In the plastids of higher plants, specific cytidines are converted into uridines (in some cases also vice versa) on the RNA level. These conversions usually restore evolutionary conserved codon identities within protein coding regions, implying that editing might be a mechanism that corrects plastid genetic information. RNA editing in general raises three main questions: what are the cis determinants defining editing sites, which factors perform the editing reaction and finally, what is the evolutionary explanation for its existence, i.e. what is its function. In recent years, plastid transformation has been used to map several cis acting elements directing the editing machinery to specific, independent sites. However, the factors mediating RNA editing remain elusive - for the moment. Now, with the recent development of a plastid in vitro editing system that made the process accessible for biochemical approaches, the identification of the participating factors is in sight. The origin and the function of plastid RNA editing, however, is still an unsolved puzzle. Here we try to give an overview on plastid RNA editing and on the efforts which shed light on this mysterious process since its discovery about a decade ago.
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