Figure 4.

Slippage activity does not disappear until a 23-nt transcript is synthesized. Transcription was carried out on the indicated templates (see Fig. 1) with ApC primers in the absence of GTP as described in Experimental Procedures. RNAs in the indicated lanes were digested with RNase T1 or chased with 200 μM of all four NTPs. Arrowheads designate the expected G-stop (nonslipped) transcripts. Slippage and leakthrough transcripts are marked by asterisks and pound signs, respectively. The percentage slippage is the fraction of complexes that underwent at least one round of transcript slippage. Lengths of selected RNAs are given in the margin of the gel.