Abstract
In recent experiments we have used the power of yeast genetics to study U1 small nuclear RNA (snRNA) : pre-messenger RNA (pre-mRNA) base pairing interactions [Séraphin étal. EMBO J. 7 (1988) 2533–2538]. Here we extend these observations to other potential U1 snRNA : pre-mRNA pairings. We show that several U1 snRNA mutants are viable. Using these Ul mutant strains we demonstrate further a basepairing interaction between U1 snRNA position 3 and intron position 6. However, this interaction is only detected with a poor splicing substrate containing branchpoint mutations. These results provide information on the mechanism of 5′ splice site-branch point interaction. We also propose several models which may explain why the sequence of the 5′ end of the U1 snRNA is conserved among organisms as divergent as man and yeast.