Evidence for a group II intron-like catalytic triplex in the spliceosome

To catalyze pre-mRNA splicing, U6 snRNA positions two metals that interact directly with the scissile phosphates. The U6 metal ligands correspond stereospecifically to metal ligands within the catalytic domain V of a group II self-splicing intron. In domain V, the ligands are organized by base-triple interactions, which also juxtapose the 3′ splice site with the catalytic metals. However, in the spliceosome, the mechanism for organizing catalytic metals and recruiting the substrate has remained unclear. Here we show by genetics, crosslinking, and biochemistry in yeast that analogous triples form in U6 and promote catalytic metal binding and both chemical steps of splicing. Because the triples include an element that defines the 5′ splice site, the triples also provide a mechanism for juxtaposing the pre-mRNA substrate with the catalytic metals. Our data indicate that U6 adopts a group II intron-like tertiary conformation to catalyze splicing.