Topological unwinding of strong and weak promoters by RNA polymerase: A comparison between the lac wild-type and the UV5 sites of Escherichia coli

Two Escherichia coli control regions have been compared in their ability to be unwound by RNA polymerase during formation of the transcriptionally active (“open”) complex: the wild-type control region, consisting of two overlapping binding sites P1 and P2, both weakly transcribed, and an “up” P1 mutant, the strong lac L8UV5 promoter. The final complexes were characterized by their topological unwinding, by DNase I and orthophenanthroline footprints, as well as by methylation of unpaired cytosine residues.