C6-G14. This pair forms a base triple with A28 via the hydroxyl group of G14. This interaction is not expected to change by mutating it to a U-A bp (SF205). The 2.5-fold decrease must therefore be a consequence of reduced stem stability and/or stacking properties with the S2 U-A pair at the junction. This large decrease again illustrates the critical importance of the A28:C6-G15 triple interaction.