Fast abortive initiation of uvrA promoter in a supercoiled plasmid studied by stopped-flow techniques.

In order to follow the fast kinetics of abortive initiation (lag time from 1 ms to 10 s), we have built a stopped-flow apparatus equipped for fluorescence detection. The small volume used for each assay (35 microliters), and the short dead time (approximately 0.5 ms) are the essential advantages of this apparatus. Supercoiling of DNA affects considerably the initiation of transcription from the uvrA promoter. It decreases the lag time due to the isomerisation process 3-fold. Nevertheless, it does not change significantly the product KBk2, which is indicative of promoter strength and shows that uvrA is an 'association-limited' promoter. The presence of the LexA repressor increases the lag time considerably. At least for small RNA polymerase concentrations this increase is stronger for supercoiled than for linearized DNA.