Abstract

In eukaryotes, RNA trans‐splicing is an important RNA‐processing form for the end‐to‐end ligation of primary transcripts that are derived from separately transcribed exons. So far, three different categories of RNA trans‐splicing have been found in organisms as diverse as algae to man. Here, we review one of these categories: the trans‐splicing of discontinuous group II introns, which occurs in chloroplasts and mitochondria of lower eukaryotes and plants. Trans‐spliced exons can be predicted from DNA sequences derived from a large number of sequenced organelle genomes. Further molecular genetic analysis of mutants has unravelled proteins, some of which being part of high‐molecular‐weight complexes that promote the splicing process. Based on data derived from the alga Chlamydomonas reinhardtii, a model is provided which defines the composition of an organelle spliceosome. This will have a general relevance for understanding the function of RNA‐processing machineries in eukaryotic organelles.