Splicing of precursor messenger RNA(pre-mRNA),involving removal of the non-coding introns and ligation of neighboring exons,is a defining feature for all eukaryotes.Aberrant splicing contributes to numerous debilitating diseases.Pre-mRNA splicing is accomplished by a dynamic supramolecular complex known as the spliceosome.A complete cycle of pre-mRNA splicing requires at least six distinct states of the spliceosome: the pre-catalytic complex(B complex),the activated complex(Bact complex),the catalytically competent complex(B* complex),the catalytic step I complex(C complex),the post-catalytic complex(P complex),and the intron lariat complex(ILS complex).In this presentation,I describe our systematic investigationinto thestructures of the yeast spliceosome using single-particle cryo-electron microscopy.A series of atomic structures identify the spliceosome as a protein-directed metalloribozyme.A portion of the U2 small nuclear RNA(snRNA)undergoes a concerted set of dazzling molecular choreography to sequentially deliver the two exon-intron junctions to the active site for the trans-esterification reactions.Remarkably,the U6 snRNA,which harbors the catalytic nucleotides,and U5 snRNA,which anchors the RNA elements of the spliceosome,remain largely static throughout the splicing cycle.