Fig. 1.

In vitro RNA substrates. The sequence and expected secondary structure of A8, A4, A3, A2, A1, A1+3, G8, and A8XL RNA are shown. Each bore a 5′-terminal triphosphate (ppp), a γ-³²P radiolabel (*) at the 5′ end, and a fluorescein label at the top of the first stem loop. Truncated derivatives of A8 (A4, A3, A2, and A1) and G8 (G4, G3, G2, G1, and G0) lacked 4–8 nucleotides from the 3′ boundary of the 5′-terminal single-stranded segment (Fig. S2). In each RNA name, the letter indicates the identity of the 5′-terminal nucleotide, and the numeral indicates the number of unpaired nucleotides at the 5′ end. A1+3 was identical to A1 except for three additional nucleotides at the 3′ end.