NUCLEAR RETENTION OF RNA AS A MECHANISM FOR LOCALIZATION

Two mutant RNAs, one derived from tRNA(i)(met), the second from U1 snR NA, that are defective in export from the nucleus to the cytoplasm hav e been studied. In both cases, the RNAs are shown to be transport comp etent but prevented from leaving the nucleus by interaction with satur able binding sites. This contradicts previous hypotheses to explain th e behavior of the tRNA mutant, and highlights a general problem in usi ng mutant RNAs to study nuclear export. In the case of these mutants, it is argued that nuclear retention is likely to be artifactual. Howev er, the additional example of U6 snRNA is described. In this case, nuc lear retention appears to be a physiological mechanism by which intran uclear localization is achieved. Evidence that the site of interaction with the La protein in U6 snRNA is important for its nuclear retentio n is presented.