Fig. 3. Characterization of the 3′–5′ exonuclease of RNAP. Autoradiograms show the effect of incubation of defined TECs carrying radiolabeled transcripts (lanes 1) under the indicated conditions. The RNA fragments are identified by their length (numbers) and 3′-terminal nucleotide (letters). The starting complexes in (A), (C) and (D) carry a ³²P label only in the 3′-terminal nucleotide, while in (B) RNA was labeled both at the 3′-terminus and internally.