Fig. 1. (A) Left: transcript sequence of the 66 bp template used in these studies (the sequence from –1 to –17 is that of a consensus T7 promoter). The invariant class II element is highlighted in magenta. Right: gel analysis of transcription on this template. Lane 1: GTP, ATP and [α-³²P]GTP only added; transcription halts at +14, misincorporation allows some transcription to +15. Lanes 2–5: UTP+CTP chase of reaction from lane 1 for 10 s to 20 min, as indicated. R.O.: runoff. (B) Kinetics of Fe-BABE transcript cleavage. Halted ECs containing a 16 nt transcript labeled with [α-³²P]CMP at its 3′-end were formed with T7RNAP conjugated with Fe-BABE at residue 388 (lane 1). Transcript cleavage was induced by addition of H₂O₂ and ascorbate to generate hydroxyl radicals either after (lane 2), simultaneous with (lane 3), or varying times before (5 s to 16 min as indicated), addition of quenching buffer.