The distinguishing feature of group II introns, and the property that links them with spliceosomal catalysis, is their ability to undergo splicing through branching. In this reaction, the 2'-hydroxyl group of a specific adenosine within intron domain 6 serves as the nucleophile for attack on the 5' splice site. We know less about branching than any other feature of group II intron catalysis, largely because the receptor structure for activating the branch site is unknown. Here, we identify the intronic region that binds the branch site of a group IIB intron. Located in domain 1, close to receptors for intron domain 5 and both splice sites, we demonstrate that the branch-site receptor is a functional element required for transesterification. Furthermore, we show that crosslinked branch sites can carry out both steps of splicing, suggesting that the conformational state of the intron core is set early and that it persists throughout the entire splicing process.