Figure 4. Removal of the O6 atoms from G2n and G3n impairs ion-induced folding of Kt-7.

The scheme at the top shows the structure of 2-aminopurine, showing that it differs from guanine by removal of O6. A 65 bp duplex with a central k-turn-containing RNA section was electrophoresed in 15% polyacrylamide in the presence of 2 mM Mg²⁺ ions. Folding of the unmodified species (tracks 2 and 6) results in a marked retardation compared with the A1nU variant (track 1). Substitution of G2n (track 4) or G3n (track 5) by 2-aminopurine lead to a marked impairment of folding (that is, increased mobility), whereas the same substitution at G1b (track 3) had a rather smaller effect.