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. 2015 May 27;5(2):1035–1062. doi: 10.3390/biom5021035

Figure 4.

Figure 4

Mechanism of transcription initiation: resolvable kinetic steps. Minimal mechanisms of open complex formation and dissociation, showing by color coding the steps that contribute to the observed rate constants (kobs, kd) for promoters like λPR that form a stable open complex RPO. For the common experimental situation of excess RNAP, the rate constant for open complex formation (kobs) is determined by the front half of the mechanism (boxed in purple), including the equilibrium constant K1 for formation of the ensemble of closed (I1) intermediates, the forward rate constant k2 of the isomerization step that includes DNA opening, and the excess RNAP concentration. Likewise, kd is determined by the late steps of the mechanism (boxed in green), including the rate constant k-2 for DNA closing and the equilibrium constant K3 for stabilization of the initial open complex I2 to form longer-lived I3 and/or RPO complexes [65,66].