Entry of RNA polymerase at the lac promoter.

The pathway of RNA polymerase entry at the lac promoter was studied by investigating the relationship between the promoter and a weak, overlapping polymerase interaction site (P2). If polymerase is made to enter the DNA by binding in vitro at this P2 site, cyclic AMP receptor protein (CRP) actively removes polymerase and redirects it to the promoter. A template competition experiment demonstrates that RNA polymerase initially bound at P2 does not slide the 22 base pairs along the DNA from this "entry" site to the promoter, but must locate the promoter by first leaving the template. We infer that CRP works by binding DNA in a way that both clears the promoter and modifies it to assume a form that is a better receptor for the binding of RNA polymerase from free solution.