Figure 3

Tetrahymena nuclease is an exonuclease. (A) Enzyme titrations (0, 0.46, and 2.3 ng) were incubated for 20 min. (on a 15% urea gel). Substrates (left to right) are Ib, IVb, S16:S33*, and S17:S34*. (B) Enzyme titration (0, 0.95, and 9.5 ng) of enriched nuclease with end-labeled non-telomeric, S1:S18 (lanes 1-3), and telomeric, S3:S20 (lanes 4-6), substrates (on a 15% urea gel). Arrow indicates an unique cleavage product of this nuclease. (C) Internally-labeled phosphorothioate substrates (with 0.9 ng of enzyme and substrates Ib, IVb, and various blockage locations on IVb, S3:S20) and (D) end-labeled phosphorothioate substrates (with 20 ng of enzyme and substrates S1:S18, S3:S20, and various single-bond blockage on S3:S20) are shown. Arrow indicates an unique cleavage product of this nuclease that is blocked by terminal phosphorothioate. Both assays were resolved on 15% urea gels.