Stalled RNA polymerase is a target of the Mfd factor

We have used a model system that made it possible to directly determine the location of stalled elongation complexes of Escherichia coli RNA polymerase on the DNA template in vivo. RNA polymerase initiates transcription from the constitutive hisR promoter within the p1EC plasmid and is halted at a downstream site by the lac repressor bound to its operator motif. RNA polymerase is halted on the plasmid within a trinucleotide (ATC)10 repeat, which allows for accurate mapping of the elongation complex on a DNA template.