Structure of a yeast activated spliceosome at 3.5 Å resolution

How spliceosomes make the first cut In eukaryotes, transcribed precursor mRNA includes noncoding sequences that must be spliced out. This is done by the spliceosome, a dynamic complex in which five small nuclear RNAs and several proteins go through a series of ordered interactions and conformational rearrangements to achieve splicing. Two protein structures provide a look at the first catalytic step in the pathway. Yan et al. report the structure of the activated spliceosome (the B act complex) at 3.5 Å resolution, revealing how latency is maintained even though the complex is mostly primed for catalysis. Wan et al. report the structure of the catalytic step 1 spliceosome (the C complex) at 3.4 Å resolution; this complex forms after the first step of the splicing reaction. Science , this issue pp. 904 and 895