Cammie Lesser (Department of Biochemistry and Biophysics, School of Medicine, University of California, San Francisco): "Nuclear pre-mRNA splicing is the process by which the introns- -the noncoding portions of precursor mRNAs--are removed and the coding sequences called exons are ligated together. This process is mediated by the spliceosome, a cellular complex composed of both RNA and proteins. A major challenge has been determining how the components of the spliceosome identify the exact intron/exon boundaries (splice-sites) and then juxtapose the catalytic residues of the spliceosome with these boundaries. The information content at the splice sites is very limited, because there is a serious problem in maintaining the fidelity of the reaction.

"Early studies demonstrated that one of the RNAs--the small nuclear RNA (snRNA) U1--base-pairs with the 5_ splice site. This RNA-RNA interaction is...