Retroelement origins of pre‐mRNA splicing

Recent cryo‐EM structures of a group II intron caught in the process of invading DNA have given new insight into the mechanisms of both splicing and retrotransposition. Conformational dynamics involving the branch‐site helix domain VI are responsible for substrate exchange between the two steps of splicing. These structural rearrangements have strong parallels with the movement of the branch‐site helix in the spliceosome during catalysis. This is strong evidence for the spliceosome evolving from a group II intron ancestor. We observe other topological changes in the overall structure of the catalytic domain V that may occur in the spliceosome as well. Therefore, studying group II introns not only provides us with insight into the evolutionary origins of the spliceosome, but also may inform the design of experiments to further probe structure–function relationships in this eukaryotic splicing apparatus. This article is categorized under: RNA Processing > Splicing Mechanisms RNA Structure and Dynamics > RNA Structure, Dynamics, and Chemistry RNA Structure and Dynamics > Influence of RNA Structure in Biological Systems RNA Evolution and Genomics > RNA and Ribonucleoprotein Evolution Intron invasion occurred during the endosymbiont event. Bacterial group II introns invaded the archaeal hostgenome. These group II introns evolved into nuclear introns and the spliceosome. This resulted in the formation of the nuclear membrane to spatially separate pre‐mRNA splicing from protein synthesis to avoid translation of intron sequences. Therefore, group II intron invasion was the defining event that catalyzed the evolution of eukaryotes.