Figure 6.

Simple repression. (a) Cis-regulatory structure of the truncated lac promoter, with the main operator O_m (yellow box) located closely downstream of the core promoter (blue box). Repressor bound at O_m will block RNAP binding to the promoter, as denoted by the overlap (green box). The DNA-binding affinity of LacI₄ for O_m is described by the dissociation constant K_m. (b) Log–log plot of the fold-change in gene expression as a function of LacI₄. Here, the repressor concentration shown on the horizontal axis refers to the cellular LacI tetramers in the absence of inducers. The experiments of Oehler et al. [27] used the operator sequences O1, O2, O3 at position O_m and measured fold-repression at two different LacI concentrations (50 nM and 900 nM); the data are shown as circles. The expected form of the fold-changes are plotted as the solid, dotted and dashed lines as indicated in the legend. The value of K_m for each curve (see legend) is determined by fitting one of the two data points. The fact that the other data point lies closely on the curve supports the applicability of the thermodynamic model to this promoter.