G4-C16. This base pair is involved in a base triple with A26, mutation of which had quite some impact on frameshifting (see above). Changing G4-C16 into A-U (SF204) is predicted to result in the loss of one hydrogen bond (G24N2H-A26N1), similar to changing A26 to G (SF218), which was still 71% active in frameshifting. The observed dramatic low activity of the A-U base pair mutant (13%) cannot be solely attributed to the loss of one hydrogen bond and must therefore also be due to compromising the stability of the S1 stem.