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.