Gene expression/regulation operon model

B. The lac operon of E coli is a good model for regulation of prokaryotic gene expression in response to environmental cues (Figure 12-4). 

The mechanisms by which operons are regulated can vary significantly from the simple model presented in Figure 28-7. Even the lac operon is more complex than indicated here, with an activator also contributing to the overall scheme, as we shall see in Section 28.2. Before any further discussion of the layers of regulation of gene expression, however, we examine the critical molecular interactions between DNA-binding proteins (such as repressors and activators) and the DNA sequences to which they bind. 

---