Thermodynamic and Kinetic Measurements of Promoter Binding by T7 RNA Polymerase
Previous steady state kinetic studies of the initiation of transcription by T7 RNA polymerase have shown that melting of the DNA helix near the transcription start site is not rate limiting (Maslak, M., & Martin, C. T. (1993) Biochemistry 32, 4281-4285). In the current work, fluorescence changes in a nucleotide analog incorporated within the promoter are used to monitor changes in the DNA helix associated with polymerase binding. The fluorescence of 2-aminopurine has been previously shown to depend on the environment of the base, with fluorescence increasing in the transition from a double stranded to a single stranded environment (Xu, D., Evans, K. O., & Nordlund, T. M. (1994) Biochemistry 33, 9592-9599). Fluorescence changes associated with polymerase binding to promoters incorporating 2-aminopurine at positions -4 through -1 support a model which includes melting, in the statically bound complex, of the region of the promoter near the start site. Equilibrium titrations at 25 ƒC with label at positionÝ-2 provide a thermodynamic measure of the dissociation constant (Kd = 4.8 nM) for promoter binding, while stopped flow kinetic assays measure the apparent association (k1 = 5.6 x 107 M-1 s-1) and dissociation (k-1 = 0.20Ýs-1) rate constants for simple promoter binding (the ratio k-1/k1 = 3.6 nM, in good agreement with the thermodynamic measurement of Kd). These results suggest that binding is close to the diffusion controlled limit and helix melting is extremely rapid. In studies of structurally altered promoters, a base functional group substitution at positionÝ-10 is shown to significantly decrease k1, with little effect on k-1. In contrast, removal of the nontemplate strand from position +1 downstream results in a large decrease in k-1, with no significant effect on k1.