FIGURE 3.

RNA pool distribution resulting from the size-independent selection. Equivalent amounts of input RNA from each round of selection were loaded on a 6% polyacrylamide gel, with RNA size markers (70, 137, and 274 nt) loaded on the right. Round 0 represents the initial recombined RNA pool. In each round, a segment of RNA from 70 nt to ~250 nt was excised from gels as indicated by the bars on the left. The 4-thiouracil incubation times from Round 1 to 4 were kept at 24 h and then decreased to 0.5 h for Round 5, and 0.05 h by Round 6. By Round 4 the RNA pool distribution showed two dominant bands, labeled upper (U) and lower (L), which were spanned by a middle region (M).