Structural Biology

Sticking Out

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Science  06 Dec 2002:
Vol. 298, Issue 5600, pp. 1849
DOI: 10.1126/science.298.5600.1849d

The eukaryotic spliceosome, a complex of RNA molecules and proteins, catalyzes the removal of noncoding regions (introns) from pre-messenger RNA (pre-mRNA). During spliceosome assembly, parts of the pre-mRNA and the U2 small nuclear RNA (snRNA) form a short, base-paired double helix in which a single unpaired adenosine, the so-called branch site, is nearly opposed to a conserved pseudouridine (ψ) residue on the U2 snRNA. The 2' OH of the unpaired adenosine acts as the nucleophile in the first cleavage reaction of splicing.

Using NMR spectroscopy, Newby and Greenbaum deter- mined the solution structures of ψ-containing and unmodified duplexes. In the ψ duplex, the branch site adenosine is extruded into the minor groove, where it makes hydrogen bonds to a Watson-Crick base pair in the helix. The backbone of the intron strand is kinked such that the 2' OH is exposed and accessible for recognition and catalysis, whereas the unmodified duplex had the branch site adenosine stacked within a continuous A-type helix. However, additional data uncovered a dynamic character in the region of the branch site, suggesting that the ψ residue helps to stabilize a structure that exists transiently in the unmodified duplex. Consistent with this, deletion of the enzyme that makes pseudouridine residues in yeast produces cells with a growth deficiency but is not lethal. — VV

Nature Struct. Biol. 10.1038/nsb873 (2002); Proc. Natl. Acad. Sci. U.S.A.99, 12697 (2002).

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