Structures of a mobile intron retroelement poised to attack its structured DNA target

Group II introns are ribozymes that catalyze their self-excision and function as retroelements that invade DNA. As retrotransposons, group II introns form ribonucleoprotein (RNP) complexes that roam the genome, integrating by reversal of forward splicing. Here we show that retrotransposition is achieved by a tertiary complex between a structurally elaborate ribozyme, its protein mobility factor, and a structured DNA substrate. We solved cryo-electron microscopy structures of an intact group IIC intronmaturase retroelement that was poised for integration into a DNA stem-loop motif. By visualizing the RNP before and after DNA targeting, we show that it is primed for attack and fits perfectly with its DNA target. This study reveals design principles of a prototypical retroelement and reinforces the hypothesis that group II introns are ancient elements of genetic diversification.

Automated summary

Group II introns are ribozymes that catalyze self-excision and function as retroelements invading DNA. In this study, cryo-electron microscopy structures were determined for a complete IIc intron maturation as a retroelement, revealing its poised integration into a DNA stem-loop motif. These findings provide insights into the design principles of retroelements and support the hypothesis that group II introns are ancient elements of genetic diversification.