Kinetic investigation of the mechanism of RNA polymerase binding to mutant lac promoters.

The initial rates of functional complex formation between Escherichia coli RNA polymerase and a family of mutant lac promoters have been determined as a function of the independently varied concentration of both components. The initial rates are first order in polymerase and zero order in promoter concentration even under conditions of polymerase excess. These data suggest that the promoter site competes with nonspecific sites for formation of an unstable complex with RNA polymerase. Whether or not a final complex is formed depends on the probability of occurrence of a conformational conversion during the lifetime of this intermediate. An important role of DNA sequence in modulating promoter binding rates is suggested: sequence changes which provide additional contacts to polymerase stabilize the intermediate and thus accelerate the rate of functional complex formation.