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Ulrich Kück, Olga Schmitt

• In eukaryotes, RNA \(\it trans\)-splicing is a significant RNA modification process for the end-to-end ligation of exons from separately transcribed primary transcripts to generate mature mRNA. So far, three different categories of RNA \(\it trans\)-splicing have been found in organisms within a diverse range. Here, we review \(\it trans\)-splicing of discontinuous group II introns, which occurs in chloroplasts and mitochondria of lower eukaryotes and plants. We discuss the origin of intronic sequences and the evolutionary relationship between chloroplast ribonucleoprotein complexes and the nuclear spliceosome. Finally, we focus on the ribonucleoprotein supercomplex involved in \(\it trans\)-splicing of chloroplast group II introns from the green alga \(\textit {Chlamydomonas reinhardtii}\). This complex has been well characterized genetically and biochemically, resulting in a detailed picture of the chloroplast ribonucleoprotein supercomplex. This information contributes substantially to our understanding of the function of RNA-processing machineries and might provide a blueprint for other splicing complexes involved in \(\it trans\)- as well as \(\it cis\)-splicing of organellar intron RNAs.