Lac operon

Francois Jacob and Jacques Monad proposed a model for an operon, which consisted of a regulator gene, an operator site consisting of a regulatory DNA sequence and one or more structural genes.

The model displayed how stimuli from the environment can promote/inhibit genetic mechanisms which control metabolic events e.g. the presence/absence of glucose and lactose in the lac operon.  The lac operon consists of an additional promoter, in front of the regulator gene, the role of which is to ensure the RNA Polymerase binds to the correct transcription initiator.

The repressor protein is a homotetramer and a product of the lacI gene, and will bind tightly to the operator, under the correct conditions i.e. when glucose is present and lactose absent. When the repressor is bound to the operon, the RNA polymerase is unable to unwind the DNA in order to expose the bases and hence is unable to transcribe the structural genes as there is no template for the RNA synthesis to occur. The group of structural genes act as a single transcription unit, coding for a single mRNA molecule termed a polycistronic transcript i.e. coding for multiple proteins and transcription is dependent upon the correct environmental conditions as described below:
In the presence of lactose, the lac operon is induced by allolactose, which binds to the lac repressor and a conformational change occurs, which results in a decreased affinity of the lac repressor for the lac operator and transcription of the structural genes occurs.
In the absence of glucose, cAMP accumulates (glucose metabolites prevent this build up when glucose is present), and cAMP is able to bind to a cAMP binding site on the lac operon activating the operon and promoting transcription.

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