EXON MUTATIONS UNCOUPLE 5' SPLICE SITE SELECTION FROM U1 SNRNA PAIRING

It has previously been shown that a mutation of yeast 5′ splice junctions at position 5 (GUAUGU) causes aberrant pre-mRNA cleavages near the correct 5′ splice site. We show here that the addition of exon mutations to an aberrant cleavage site region transforms it into a functional 5′ splice site both in vivo and in vitro. The aberrant mRNAs are translated in vivo. The results suggest that the highly conserved G at the 5′ end of introns is necessary for the second step of splicing. Further analyses indicate that the location of the U1 snRNA-pre-mRNA pairing is not affected by the exon mutations and that the precise 5′ splice site is selected independent of this pairing. © 1990.