Abstract
In eukaryotes, RNA 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 trans-splicing have been found in organisms within a diverse range. Here, we review 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 trans-splicing of chloroplast group II introns from the green alga 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 trans- as well as cis-splicing of organellar intron RNAs.
Highlights
One of the unexpected and outstanding discoveries in 20th century biology was the identification of discontinuous eukaryotic genes [1,2]
Spliceosomal introns and group II introns share a common splicing mechanism that is distinctly different from the splicing mechanism of group I introns, which are found in genomes of bacteria and organelles as well as in nuclei of lower eukaryotes
We describe nuclear and organellar splicing factors further and discuss how trans-spliced and degenerated group II introns may have evolved into spliceosomal introns
Summary
One of the unexpected and outstanding discoveries in 20th century biology was the identification of discontinuous eukaryotic genes [1,2]. The intervening sequences in the nuclear precursor mRNA of eukaryotes are described as spliceosomal introns or nuclear mRNA introns. Further intron types have been identified and are classified by their structural characteristics and splicing mechanisms, i.e., archaeal introns, group I introns, and group II introns. Spliceosomal introns and group II introns share a common splicing mechanism (as outlined later) that is distinctly different from the splicing mechanism of group I introns, which are found in genomes of bacteria and organelles as well as in nuclei of lower eukaryotes. Archaeal introns are comparably small and share a pre-tRNA intron excision mechanism with eukaryotes. We describe nuclear and organellar splicing factors further and discuss how trans-spliced and degenerated group II introns may have evolved into spliceosomal introns.
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