Polymerase selectivity and the promoters of U snRNA genes

Capítulo en: Fritz Eckstein; David M. J. Lilley (eds.). Nucleic Acids and Molecular Biology. Berlin: Springer, 1992, p.174-186. (Nucleic Acids and Molecular Biology ; 6)The U small nuclear RNAs (snRNAs) form a functionally conserved family of RNAs found in eukaryotic cells. A subset of these RNAs, the spliceosomal snRNAs, function in the removal of introns from messenger RNA precursors. In most eukaryotes this group of snRNAs are U1, U2, U4, U5 and U6 but in trypanosomes, which produce mature mRNAs by transrather than cis-splicing, no homologues of U1 or U5 have been found (see Guthrie and Patterson 1988; Lamond et al. 1990 for reviews). Another ubiquitous member of the U snRNA family which will be discussed here is U3. U3 is found in the nucleolus rather than the nucleoplasm and is involved in the processing of ribosomal RNA precursors (Kass et al. 1990). The topic of this review will not, however, be the function of these RNAs, what will interest us is their transcriptionPeer Reviewe

Importin provides a link between nuclear protein import and U snRNA export

Importin-alpha mediates nuclear protein import by binding nuclear localization signals and importin-beta. We find approximately 30% of SRP1p, the yeast importin-alpha, in a nuclear complex with the Saccharomyces cerevisiae nuclear cap-binding protein complex (CBC). Similarly, a large fraction of Xenopus CBC is associated with importin-alpha in the nucleus. CBC promotes nuclear export of capped U snRNAs and shuttles between nucleus and cytoplasm. The CBC-importin-alpha complex binds specifically to capped RNA, suggesting that CBC might shuttle while bound to importin-alpha. Strikingly, importin-beta binding displaces the RNA from the CBC-importin-alpha complex. Thus, the commitment of CBC for nuclear reentry triggers the release of the export substrate into the cytoplasm. We provide evidence for a mechanism that ensures that importin-mediated RNA release is a specifically cytoplasmic event

Nucleocytoplasmic Distribution of Human RNA-editing Enzyme ADAR1 Is Modulated by Double-stranded RNA-binding Domains, a Leucine-rich Export Signal, and a Putative Dimerization Domain

The human RNA-editing enzyme adenosine deaminase that acts on RNA (ADAR1) is expressed in two versions. A longer 150-kDa protein is interferon inducible and can be found both in the nucleus and cytoplasm. An amino-terminally truncated 110-kDa version, in contrast, is constitutively expressed and predominantly nuclear. In the absence of transcription, however, the shorter protein is also cytoplasmic and thus displays the hallmarks of a shuttling protein. The nuclear localization signal (NLS) of human hsADAR1 is atypical and overlaps with its third double-stranded RNA-binding domain (dsRBD). Herein, we identify regions in hsADAR1 that interfere with nuclear localization and mediate cytoplasmic accumulation. We show that interferon-inducible hsADAR1 contains a Crm1-dependent nuclear export signal in its amino terminus. Most importantly, we demonstrate that the first dsRBD of hsADAR1 interferes with nuclear localization of a reporter construct containing dsRBD3 as an active NLS. The same effect can be triggered by several other, but not all dsRBDs. Active RNA binding of either the inhibitory dsRBD1 or the NLS bearing dsRBD3 is required for cytoplasmic accumulation. Furthermore, hsADAR1's dsRBD1 has no effect on other NLSs, suggesting RNA-mediated cross talk between dsRBDs, possibly leading to masking of the NLS. A model, incorporating these findings is presented. Finally, we identify a third region located in the C terminus of hsADAR1 that also interferes with nuclear accumulation of this protein