Cryptic branch point activation allows accurate in vitro splicing of human beta-globin intron mutants

The excised introns of pre-mRNAs and intron-containing splicing intermediates are in a lariat configuration in which the 5' end of the intron is linked by a 2'-5' phosphodiester bond (RNA branch) to a single adenosine residue near the 3' end of the intron. To determine the role of the specific sequence surrounding the RNA branch, we have mutated the branch point sequence of the human beta-globin IVS1. Pre-mRNAs lacking the authentic branch point sequence are accurately spliced in vitro; processing of the mutant pre-mRNAs generates RNA lariats due to the activation of cryptic branch points within IVS1. The cryptic branch points always occur at adenosine residues, but the sequences surrounding the branched nucleotide vary. Regardless of the type of mutation or the sequences remaining within IVS1, the cryptic branch points are 22 to 37 nucleotides upstream of the 3' splice site. These results suggest that RNA branch point selection is primarily based on a mechanism that measures the distance from the 3' splice site.