G3 The lac operon

The lacl gene has its own promoter (Pkd) to which RNA polymerase binds and initiates transcription. In the absence of an inducer, the lacl gene is transcribed, producing lac repressor mRNA and hence lac repressor protein monomers. These monomers assemble to form active tetramers which bind to the lac operator site, 0, , and prevent transcription of the lac operon. In the presence of an inducer (such as allolactose or IPTG), the inducer binds to the repressor and changes its conformation, reducing its affinity for the lac operator. Thus the repressor now dissociates and allows RNA polymerase to transcribe the lac operon. 

Catabolite activator protein, CAP (also called cAMP receptor protein, CRP) is an activator required for high level transcription of the lac operon. The active molecule is a CRP dimer that binds 3 5 cyclic AMP to form a CRP-cAMP complex. CRP-cAMP binds to the lac promoter and increases the binding of RNA polymerase, stimulating transcription of the lac operon. CRP dimer without cAMP cannot bind to this DNA. The action of CRP depends upon the carbon source available to the bacterium. When glucose is present, the intracellular level of cAMP falls, CRP cannot bind to the lac promoter and the lac operon is only weakly transcribed. When glucose is absent, the level of intracellular cAMP rises, the CRP-cAMP complex stimulates transcription of the lac operon and allows lactose to be used as an alternative carbon source. 

Instant Notes in Biochemistry 2nd Edition, B.D. Hames N.M. Hooper, (c) 2000 BIOS Scientific Publishers Ltd, Oxford. 

It was Jacob and Monod in 1961 who proposed the operon model for the regulation of transcription. The lac operon is a good example of how operons work (Fig. 1). The operon model proposes three elements  

The lacl gene has its own promoter (PkI) that binds RNA polymerase and leads to transcription of lac repressor mRNA and hence production of lac repressor protein monomers. Four identical repressor monomers come together to form 

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