Figure 9. Drugs that inhibit elongation affect the kinetics of RNA synthesis in specific ways.

Drugs were added to doxycycline-activated cells and GFP-MS2 transcription sites were photobleached. (a) Fluorescence recovery after actinomycin D treatment (5 µg ml⁻¹) for 20 min resulted in a large immobile fraction, indicating stalling of the polymerase owing to intercalation. (b) Fluorescence recovery in untreated cells (normal) and cells treated with the fast-acting drug camptothecin (14 µM) for 15–45 min (+Cmt), detected with a Zeiss confocal microscope (see Supplementary Methods). Treatment with camptothecin led to a much slower recovery. These data are consistent with the drug causing inhibition of topoisomerase I, so that polymerases can not proceed at full speed owing to torsional stress imposed by the supercoiling of the DNA⁴¹. Error bars in a,b show s.e.m.