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Repressor Recognition of Operator and Effectors

Mary D. Barkley, Suzanne Bourgeois


As predicted by Jacob and Monod (1961), negative control of the lac operon is accomplished by the lac repressor protein, the operator site on the bacterial chromosome, and small effector ligands. This paper deals with two unique attributes of the repressor protein: its ability to recognize the lac operator and its ability to bind inducer. The lac repressor (R) binds at the operator site (O) on DNA with very high affinity:(1)R+O⇌RO.

This simple stoichiometry reflects the observation that one repressor molecule binds one operator region.

There are two classes of ligands which affect the interaction of repressor protein and operator DNA: inducers and anti-inducers. These low-molecular-weight sugar molecules, primarily β-galactosides, can bind to repressor protein whether it is free in solution or bound to DNA. There are four sites for effector (E) per repressor tetramer, one per subunit:(2)R+4E⇌RE4.

Inducing and anti-inducing ligands interact directly with repressor protein bound to the operator, forming a ternary complex and decreasing or increasing, respectively, the affinity of repressor for operator. Binding of one inducer molecule to repressor-operator complex apparently releases the repressor from the operator:(3)RO+E⇌ERO⇌RE+O.

The binding sites on the protein for DNA and for effectors are separate sites; this is well documented by genetic evidence presented in this book and biochemical evidence discussed in this paper. However, these sites are mutually interacting: binding of effectors alters the affinity of repressor for operator, and vice versa. The interplay between the DNA- and effector-binding sites is mediated...

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