The Lac Operon

The lactose operon (Figure 4-1) provides a good model system of several concepts of prokaryotic gene regulation. It consists of three structural genes (lacZ, lacY, and lacA) as well as three controlling sites (lacCRP, lacP1, and lacO). The structural genes lacZ, lacY, and lacA encode the enzymes β-galactosidase, permease, and transacetylase, respectively. Catabolism of lactose is dependent upon these proteins. The controlling sites lacCRP, lacP1, and lacO are binding sites for the cAMP receptor protein, RNA polymerase, and the lactose repressor, respectively.

The lactose regulatory operon consists of one structural gene (lacI) and one controlling site (lacP2). The structural gene encodes the lactose repressor, whereas the controlling site is an RNApolymerase binding site (a promoter).

In the absence of inducer, something that turns on transcription of an operon, expression is inhibited by the binding of the lactose repressor at lacO. The repressor sterically hinders the binding of RNApolymerase to lacP1 and the inititaion of transcription. If lactose is present, it can be converted by the cell into allolactose, which acts as an inducer for this operon. When inducer is present, it binds to the repressor (protein lacI) and inactivates it. Inactive lacI cannot bind the operator, and RNApolymerase is able to bind to lacP1 and initiate transcription of the genes necessary for lactose catabolism.

Even when an operon is induced the cell does not rapidly fill with mRNAand protein since mRNA has an average half-life of only 2.5 minutes. That means that 2.5 minutes after mRNA is synthesized, half of it will be degraded. Proteins are more stable than mRNA. Also, as the cell synthesizes mRNA and proteins, it depletes its stores of certain energy molecules. When a cell is rapidly metabolizing, catabolite repression shuts down many catabolic operons, including the lactose operon. This involves a small molecule called cyclic adenosine monophosphate (cAMP). The cellular level of cAMP decreases when the synthesis of lactose mRNA and enzymes increases, and the level of cAMPincreases when these catabolic genes are no longer expressed. The higher the cAMP level, the more cAMP binds to a protein called cyclic AMP receptor protein (CRP), which then undergoes a conformational change that promotes binding to an activator binding site (lacCRP). This in turn upregulates transcription of the lactose operon genes.

Notes
Because the lactose operon is repressed by a regulatory protein, it is said to be negatively controlled. Other operons are positively controlled, that is regulated by proteins that activate the operon.

Lactose present = operon on Lactose absent = operon off