Crystal structure of a group II intron in the pre-catalytic state.

Group II introns are self-splicing catalytic RNAs that are thought to be ancestral to the spliceosome. Here we report the 3.65-Å crystal structure of the group II intron from Oceanobacillus iheyensis in the pre-catalytic state. The structure reveals the conformation of the 5' splice site in the catalytic core and represents the first structure of an intron prior to the first step of splicing.