Association with terminal exons in pre-mRNAs: a new role for the U1 snRNP?

Psoralen cross-linking experiments in HeLa cell nuclear extracts have revealed the binding of U1 snRNA to substrates containing the SV40 late and adenovirus L3 polyadenylation signals. The sites of U1 cross-linking to the substrates map different distances upstream of the AAUAAA sequence to regions with limited complementarity to the 5' end of U1 snRNA. U1 cross-linking to the same site in the SV40 late pre-mRNA is enhanced by the addition of an upstream 3' splice site, which also enhances polyadenylation. Examination of different nuclear extracts reveals a correlation between U1 cross-linking and the coupling of splicing and polyadenylation, suggesting that the U1 snRNP participates in the coordination of these two RNA-processing events. Mutational analyses demonstrate that U1/substrate association cannot be too strong for coupling to occur and suggest that the U1 snRNP plays a similar role in recognition of internal and 3' terminal exons. Possible mechanisms for communication between the splicing and polyadenylation machineries are discussed, as well as how interaction of the U1 snRNP with 3' terminal exons might contribute to mRNA export.