Precatalytic Intron Structure Solved

The first crystal structure of the precatalytic state of a group II intron has been solved (Nat. Struct. Mol. Biol., DOI: 10.1038/nsmb.2270). Previous structures of these self-splicing RNA molecules, which are thought to be evolutionary predecessors of the spliceosome, have captured them either during or after the splicing process, but never before splicing starts. Navtej Toor of the University of California, San Diego, and coworkers obtained a 3.65-Å crystal structure of the group II intron from the ocean sediment bacterium Oceanobacillus iheyensis in its precatalytic state. To trap this state, the researchers deactivated the catalytic site with a single-point mutation. The structure shows that the 5ʹ splice site has a kinked structure before catalysis, which had been predicted. The researchers propose that this structural distortion of splice sites is a general mechanism inherent to all RNA-splicing reactions. They also modeled the probable position of the 3ʹ splice site and used this information to propose a mechanism for the entire splicing pathway. Their model suggests that the active site can accommodate both splicing steps with relatively small conformational alteration.