The invariant U5 snRNA loop 1 sequence is dispensable for the first catalytic step of pre-mRNA splicing in yeast

An in vitro reconstitution system was developed to study the role of U5 snRNA in the two catalytic steps of pre-mRNA splicing. It is known that the invariant loop 1 sequence of U5 interacts with exon sequences at the 5' splice location before the first catalytic step. An in vitro analysis of U5 mutations shows that the first transesterification precisely occurs when the U5 loop is absent. Thus, this sequence is not a major component of the spliceosomal active site for the first catalytic step. The second catalytic step was found to tolerate mutations in the invariant sequence.