Susan J. Baserga

Associate Professor of Therapeutic Radiology and Genetics

B.S. Yale College, 1980
M.D., Ph.D. Yale University, 1988

Research Interests:

RNA Transport, Metabolism
Molecular Mechanisms of Gene Expression
Hemoglobin

RNA transport between the nucleus and the cytoplasm is an important modulator of gene expression. Since some small nuclear RNAs (snRNAs) follow unique pathways, we are studying one of them to define the RNA and protein components required for transport. In a related area of research, we are studying the role that nonsense mutations in mRNAs play in RNA transport and metabolism.

Current Research

The U3 snRNA, found in the nucleolus, participates in processing of the nascent rRNA transcript. Using microinjection into the Xenopus oocyte we have found that upon transcription, the U3 snRNA exits to the cytoplasm. There its cap structure undergoes hypermethylation and it can bind the protein fibrillarin. However, unlike the other snRNAs so far studied, neither its cap structure nor the fibrillarin protein are necessary for import back into the nucleus. Instead, import depends on the maintenance of a structural element at its 3' end: a short 3' terminal stem. We predict that a protein important for import may be binding in the loop. Studies are underway to identify the proteins complexed with U3 which are necessary for import back into the nucleus. A battery of 20 mutant U3 RNAs is being studied with respect to fibrillarin binding to define, upon injection into Xenopus oocytes, the sequence or structure that is recognized. Furthermore, since U3 function in Xenopus oocytes can be abolished by the injection of antisense oligonucleotides to U3, rescue by injection of mutant U3 snRNAs will elucidate the role of specific U3 sequences and structures in rRNA processing.

Nonsense mutations in the first two exons of the beta-globin mRNA affect its nuclear metabolism and transport. We are beginning studies to define the connection between exon definition in mRNA splicing and the effect of nonsense mutations on mRNA metabolism.

(A) U3 RNA requires a structural motif, the 3'-terminal stem, for nuclear import. In vitro-transcribed U3 RNA and two RNAs with mutations in the 3'-terminal stem were injected into Xenopus oocytes. The U3 stem mutation abolishes the 3'-terminal stem; the U3 stem suppressor mutation restores the 3'-terminal stem with compensatory mutations in the base-paired strand. Oocytes were fixed and dissected into cytoplasm (c) and nucleus (n), either immediately after injection (c0, n0) or after 18 hr (c18, n18). RNA from each compartment was analyzed on an 8% denaturing polyacrylamide gel. (B) U3 RNA requires a 3'-terminal stem for TMG cap formation. In vitro-transcribed U3 RNA or two U3 RNAs with mutations in the 3'-terminal stem (stem and stem suppressor) were injected into Xenopus oocytes. Total RNA was harvested at 18 hr. Equal amounts of 32P-labeled RNA were immunoprecipitated with TMG antibodies and analyzed on an 8% denaturing polyacrylamide gel.

Representative Publications:

Lee, S.J. and Baserga, S.J. Functional separation of pre-rRNA processing steps revealed by truncatio of the U3 small nucleolar ribonucleoprotein component, Mpp10. Proc. Natl. Acad. Sci. USA 94:13536-13541 (1997).

Westendorf,J.M., Konstantin,K.N., Wormsley,S., Shu,M.D., Matsumoto-Taniura,N., Pirollet,F., Klier,F.G., Gerace,L., and Baserga,S.J. M phase phosphoprotein 10 (MPP10) is a human U3 snoRNP component. Mol.Biol.Cell 9:437-449 (1998).

Lee,S.J. and Baserga,S.J. Imp3p and Imp4p: two specific components of the U3 small nucleolar ribonucleoprotein that are required for pre-18S rRNA processing. Mol. Cell. Biol. 19:5441-5442 (1999).

Lyman,S.K., Gerace,L. and Baserga,S.J. Human Nop5/Nop58 is a component common to the box C/D small nucleolar ribonucleoproteins. RNA 5:1597-1604 (1999).

Dunbar,D.A., Wormsley,S., Lowe,T. and Baserga,S.J. Trypanosoma brucei has many Box C/D small nucleolar RNAs with the potential to guide 2ā-0-ribose methylation of rRNA. J. Biol. Chem. 275 :14767-14776 (2000).

Dunbar, D.A., Chen, A.A., Wormsley, S. and Baserga, S.J. The genes for small nucleolar RNAs in Trypanosoma brucei are organized in clusters and are transcribed as a polycistronic RNA. Nuc. Acids Res. 28: 2855-2861 (2000).

Updated 30 August 2000