U6 snRNA maturation and stability.

The U6 snRNP is found as a monomer and as a heterodimer, complexed with the U4 snRNP (U4/U6). Northern blotting detects approximately equal amounts of U4/U6 heterodimer and U6 monomer in the nucleus but only U6 monomer in bona fide cytoplasm. In mammalian cells, newly synthesized U6 appears transiently in the cytoplasm before returning to the nucleus. Sedimentation analysis identifies cytoplasmic U6 in similarly sized structures as nuclear U4 and U6 and smaller structures than cytoplasmic U4. Inhibitor studies demonstrate that newly synthesized U6 can move from the cytoplasm into the nucleus in the absence of U4 synthesis. The nuclear half-life of U6 is significantly shorter than that of U4 and the other spliceosomal snRNAs. These data support a model in which U4 and U6 snRNAs undergo distinct cytoplasmic maturation pathways before returning to the nucleus, where the U4/U6 snRNP assembles.