Activation of a cryptic 5 ' splice site by U1 snRNA

In the course of analyzing 5 ' splice site mutations in the second intron o f Schizosaccharomyces pombe cdc2, we identified a cryptic 5 ' junction cont aining a nonconsensus nucleotide at position +2. An even more unusual featu re of this cryptic 5 ' junction was its pattern of activation. By analyzing the profile of splicing products for an extensive series of cdc2 mutants i n the presence and absence of compensatory U1 alleles, we have obtained evi dence that the natural 5 ' splice site participates in activation of the cr yptic 5 ' splice site, and that it does so via base pairing to U1 snRNA, Fu rthermore, the results of follow-up experiments strongly suggest that base pairing between U1 snRNA and the cryptic 5 ' junction itself plays a domina nt role in its activation. Most remarkably, a mutant U1 can activate the cr yptic 5 ' splice site even in the presence of a wild-type sequence at the n atural 5 ' junction, providing unambiguous evidence that this snRNA redirec ts splicing via base pairing, Although previous work has demonstrated that U5 and U6 snRNAs can activate cryptic 5 ' splice sites through base pairing interactions, this is the first example in which U1 snRNA has been implica ted in the final selection of a cryptic 5 ' junction.