RNA substrates used. (A) Sequence of "Symmetrical/Substrate U". Commercially synthesized RNA 10 mers were prepared and phosphorylated with isotopic [γ32P] label (*) as described in Methods. After melting and annealing, the RNA molecules self-anneal to form a six base-pair duplex with identical 5' overhangs. For Sym/Sub U, the correct base at N + 1 position, A, and one incorrect base, U, is shown (incorporated bases shown in lighter type). (B) List of the four Sym/Subs used in this study. Four similar synthetic RNAs were used. The first template base is shown in bold, and the correct base indicated in the right-hand column.