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(A)
(B)
FIGURE 1.2
Panel A. The lac repressor protein in action. The lac repressor protein binds the lac operon
at the operator, preventing transcription of the lac operon messenger RNA. The operon is
Off. Panel B. Binding of allolactose to the lac repressor causes a conformational change in
the repressor, preventing it from binding at the operator. Transcription of the lac operon
messenger RNA can proceed. The Operon is On. Figure reproduced from CBE-Life Sci-
ences Education , Vol. 9, Fall 2010, p. 233.
won't bind at the lac promoter, and the lac mRNA is not made. No mRNA means no
β
-galactosidase or lactose permease, and the operon will be off. 1 The key to the entire
system is cAMP. If glucose is present, E. coli uses the glucose and the catabolism of
glucose keeps cAMP levels low. After the glucose is used up, cAMP levels rise. The
CAP protein binds cAMP, and the CAP-cAMP complex binds the DNA and facilitates
the attachment of RNA polymerase. Figure 1.3 presents a schematic of the whole lac
operon regulatory mechanism.
1.3 BOOLEAN NETWORK MODELS OF THE LAC OPERON
In this section we design some basic dynamical models of the lac operon mechanism,
capable of reflecting its biological behavior. At the very minimum, a model should
1 To be more exact, when both lactose and glucose are present, some small amounts of mRNA,
β
-galactosidase, and lactose permease will always be made (since the repressor protein will be blocked
from binding to the operator site). However, those levels will be thousands of times lower than they
would be in the absence of glucose.
 
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