Question

Inside E. Coli there are about 10^-8 M of Lac repressor, and 1 binding site on the bacterial genome. The equilibrium constant K for binding of the repressor to the binding site is 10^13 M^-1.

What fraction of the binding site would you expect to be bound by the repressor?​

Answer 1

Answer:

The lac operon is negatively regulated by a repressor, the product of the lacI gene (additional positive aspects of lac regulation result from action of cAMP-CAP). The lac repressor binds to a specific DNA sequence called the operator (lacO) and prevents efficient initiation of transcription by RNA polymerase from the promoter (lacP). An inducer (allolactose or an analog) binds to the repressor and prevents its binding to the operator, thereby releasing the repression and allowing transcription of the lac operon.

a) Most mutations in the operator, the binding site for repressor, lead to lower affinity for the repressor and hence less binding. Thus these mutations allow continued transcription (and thus expression) of the lac operon even in the absence of inducer; this is referred to constitutive expression.

b) Mutations in the repressor that prevent its binding to the operator will lead to constitutive expression (no repression in the absence of inducer). Mutations that prevent binding of the inducer without affecting the ability to bind to the operator lead to a non‑inducible phenotype.

c) The lac promoter is not a particularly strong promoter, and mutations have been obtained that either increase or decrease its efficiency of initiating transcription. Base substitutions that make the promoter sequence more similar to the consensus generate a stronger promoter (promoter "up" mutations) whereas those that make the promoter less similar to the consensus generate a weaker promoter (promoter "down" mutations). An "up" mutation would make the lac operon no longer dependent on the positive regulation by the cAMP‑CAP complex (when the operon is induced). A "down" mutation would not allow expression even in the de‑repressed state (presence of inducer) and hence would show a non‑inducible phenotype.