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Putative components of the spliceosomal active site include a bulged helix between U2 and U6 snRNAs (U2-U6 helix I) and the adjacent ACAGAG hexanucleotide in U6. We have developed an in vivo, bimolecular randomization-selection method to functionally dissect these elements. Although a portion of U2-U6 helix I resembles the G-binding site of group I introns, the data are inconsistent with an analogous functional role for this structure in the spliceosome. Instead, analysis of several novel covariants supports the existence of a structure in which the helix I bulge engages in a tertiary interaction with the terminal residue of the U6 hexanucleotide (ACAGAG). Such a higher order structure, together with other known interactions, would juxtapose the two clusters of residues of the U2-U6 complex that are specifically required for the second chemical step of pre-mRNA splicing with the 3' splice site. Indeed, mutations in the residues that participate in the tertiary interaction affect both the efficiency and fidelity of 3' splice site usage.

Randomization-selection analysis of snRNAs in vivo: evidence for a tertiary interaction in the spliceosome.