Fig. 2. RNA polymerase II must be in an elongation complex to cleave nascent transcripts.

Elongation complexes containing ³²p-labeled runoff RNA (RO), transcript Ia, and the 325-nucleotide RNA were synthesized as described in the legend of Fig. 1. The RNAs were isolated and run on a polyacrylamide gel (lane 1) or mixed with intact, washed elongation complexes (lane 2). This mixture, and washed elongation complexes alone (lanes 3 and 4), were incubated with 7 mM MgCl₂ and 2 μg of partially purified bovine brain SII at 28 °C for 15 min. One reaction (lane 4) also received α-amanitin (1 μg/ml) before incubation. RNA was isolated and analyzed by electrophoresis as described under “Materials and Methods.” Migration positions of RNAs are indicated as described in the legend of Fig. 1.