Mutant U2 snRNAs of Xenopus which can form an altered higher order RNA structure are unable to enter the nucleus.

We studied the nuclear targeting of U snRNAs by microinjection of wild-type and mutant U2 small nuclear RNA transcripts into the cytoplasm of Xenopus oocytes. It has previously been shown that a mutant U2 RNA (delta C) which does not bind certain common U snRNP proteins, some of which carry epitopes recognized by anti-Sm antisera, does not enter the nucleus. We show here that several mutant U2 RNAs which bind to Sm antigens do not enter the nucleus, demonstrating that this RNA-protein interaction is insufficient to produce a nuclear targeting signal. Computer predictions of the secondary structures of the RNAs, derived from minimal energy calculations, show that those which are unable to enter the nucleus have the potential to form an additional secondary structure interaction due to base complementarity between sequences near to their 5' and 3' ends. The data suggest that this structural feature inhibits nuclear targeting.