snRNA Catalysts in the Spliceosome’s Ancient Core

The spliceosome, an assembly of snRNAs and proteins, catalyzes the removal of introns from premessenger RNAs. A new study identifies specific phosphates in the U2-U6 snRNA complex that position two catalytic metals. Remarkably, these correspond precisely to metal-binding phosphates in a homologous structure of Group II self-splicing introns, long proposed to be the ribozyme progenitor of spliceosome.