A point mutation in the conserved hexanucleotide at a yeast 5' splice junction uncouples recognition, cleavage, and ligation.

We have constructed an actin-HIS4 gene fusion, such that expression of HIS4 requires proper splicing of the actin intron. Using this chimeric gene in an in vivo screen for splicing mutations, we have isolated a G to A transition in the fifth position of the yeast 5' consensus sequence/GTAPyGT. This mutation still allows the junction to be recognized by the splicing machinery, albeit inefficiently. Surprisingly, the fidelity of the 5' endonucleolytic cleavage is also reduced. This results in an incorrect cleavage 6 nucleotides 5' of the 5' junction, at the dinucleotide/AT. Cleavage at this abnormal site does not lead to the production of mature mRNA, although this species appears to be in a lariat structure. The behavior of this mutant argues that recognition of the 5' junction and subsequent cleavage are separable events and, furthermore, that requirements for 3' endonucleolytic cleavage may be more complex than previously imagined.