A ribozyme selected from variants of U6 snRNA promotes 2 ',5 '-branch formation

In vitro selection was used to sample snRNA-related sequences for ribozyme activities, and several 2',5'-branch-forming ribozymes were isolated. One s uch ribozyme is highly dependent upon an 11-nt motif that contains a conser ved U6 snRNA sequence (ACAGAGA-box) known to be important for pre-mRNA spli cing. The ribozyme reaction is similar to the first step of splicing in tha t an internal 2'-hydroxyl of an unpaired adenosine attacks at the 5'-phosph ate of a guanosine. It differs in that the leaving group is diphosphate rat her than a 5' exon. The finding that lariat formation can be accomplished b y a small RNA with sequences related to U6 snRNA indicates that the RNA ava ilable in the spliceosome may be involved in RNA-catalyzed branch formation .