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The invariant AGC triad of U6 snRNA plays an essential, unknown role in splicing. The triad has been implicated in base-pairing with residues in U2, U4, and U6. Through a genetic analysis in S. cerevisiae, we found that most AGC mutants are suppressed both by restoring pairing with U2, supporting the significance of U2/U6 helix Ib, and by destabilizing U2 stem I, indicating that this stem regulates helix Ib formation. Intriguingly, one of the helix Ib base pairs is required specifically for exon ligation, raising the possibility that the entirety of helix Ib is required only for exon ligation. We also found that U4 mutations that reduce complementarity in U4 stem I enhance U2-mediated suppression of an AGC mutant, suggesting that U4 stem I competes with the AGC-containing U4/U6 stem I. Implicating an additional, essential function for the triad, three triad mutants are refractory to suppression--even by simultaneous restoration of pairing with U2, U4, and U6. An absolute requirement for a purine at the central position of the triad parallels an equivalent requirement in a catalytically important AGC triad in group II introns, consistent with a role for the AGC triad of U6 in catalysis.

Multiple functions for the invariant AGC triad of U6 snRNA