Involvement of U6 snRNA in 5' splice site selection.

Two models describing the interaction between U6 small nuclear RNA (snRNA) and the 5' splice site of introns have been proposed on the basis of cross-linking experiments. Here it is shown that a conserved sequence present in U6 snRNA forms base pairs with conserved nucleotides at the 5' splice junction and that this interaction is involved in 5' splice site choice. These results demonstrate a specific function for U6 snRNA in splicing and suggest that U6 snRNA has a proofreading role during splice site selection. A model is presented in which this new interaction, in concert with previously described interactions between U6 snRNA, U2 snRNA, and the pre-messenger RNA, would position the branch point near the 5' splice site for the catalysis of the first splicing step.