Figure 7. A strategy for distinguishing two different mechanisms of transcriptional activation.

(A) A TF can activate transcription in two ways: by stabilizing the RNAP-DNA complex or by accelerating the rate at which this complex initiates transcripts. (B) A thermodynamic model for the dual mechanism of transcriptional activation illustrated in panel A. Note that $\alpha$ multiplies the Boltzmann weight of the doubly bound complex, whereas $\beta$ multiplies the transcript initiation rate of this complex. (C) Data points measured as in Figure 4C will lie along a 1D allelic manifold having the form shown here. This manifold is computed using $t_{+}$ values from Equation 7 and $t_{-}$ values from Equation 2. Note that regime five occurs at a point positioned ${\beta }^{\prime }$-fold above the diagonal, where ${\beta }^{\prime }$ is related to $\beta$ through Equation 8. Measurements in or near the strong promoter regime (${\displaystyle P\gtrsim 1}$) can thus be used to determine the value of ${\beta }^{\prime }$ and, consequently, the value of $\beta$. (D) The five regimes of this allelic manifold are listed.