*2.2. RNA Metabolism*

Most chloroplast genes in plants are organized as operons. These polycistronic primary RNAs require extensive processing, including 5 and 3 trimming, intercistronic cleavage, RNA splicing, and RNA editing [39]. Evidence suggests that 5 and 3 trimming and intercistronic cleavage are important for moderating RNA stability and translation within chloroplasts [40–44]. In plants, approximately 20 chloroplast genes (encoding proteins or structural RNAs) are interrupted by introns. RNA splicing removes the intron sequences of genes from primary transcripts to enable the production of mature mRNA with the correct genetic information [45,46]. RNA analyses have shown that RNA editing (mainly in the form of C-to-U base conversions) is highly widespread within the chloroplasts of land plants. During this process, numerous C-to-U conversions alter the coding sequences of chloroplast mRNAs, regulate RNA secondary structures that influences the splicing and/or stability of RNAs, or generate translational start sites (AUG) [47,48]. All of these RNA metabolic events depend on many nucleus-encoded proteins, most of which likely arose during coevolution between the host and endosymbiont. For detailed information on chloroplast RNA metabolism, we direct the reader to recent reviews in this area [39,43,45,48,49].
