Mutations can affect the regulation of the lac operon. For example, the lac I gene might become mutated such that the repressor encoded no longer binds to lactose. In this case, the repressor would bind to the operator regardless of the presence or absence of lactose, and the operon would never be transcribed at high levels.
Alternatively, the lac I gene could be mutated such that the repressor no longer bound to the operator. In this case, the operon would never be repressed, and transcription would be carried out continuously. This is known as constitutive transcription.
Constitutive transcription can occur with another type of mutation. If the operator region is mutated such that the wild-type repressor doesn't recognize it (the repressor recognizes the specific DNA sequence of the operator region), there will be no binding of the repressor to the operator, and transcription will go on continuously.
Catabolite Repression
Expression of the lac operon can be regulated another way. Glucose is preferable to lactose as an energy source, so if glucose is present in the environment, the lac operon is down-regulated (transcription is reduced). Here is how it works:
Although it wasn't mentioned previously, transcription of the lac operon requires another protein, called catabolite activator protein (CAP for short). This CAP protein binds to the lac promoter and promotes transcription, but only after it has bound to a small molecule called cyclic AMP (cAMP). Without cAMP, CAP will not bind to the promoter, and no transcription will occur. In the previous examples involving the lac operon, we can assume that cAMP was present, and the CAP-cAMP complex was bound to the promoter.
cAMP is produced by an enzyme called adenylcyclase. When bacteria encounter glucose in the environment, adenylcyclase is inhibited, and cAMP production drops. There is no cAMP to bind to CAP, so CAP will not bind to the lac promoter, and no lac transcription takes place. In this way, the bacterium does not produce enzymes for lactose metabolism when they are not necessary because of the presence of glucose. This also offers a form of feedback inhibition. Beta-galactosidase (the lac Z gene product) breaks lactose down to glucose and galactose, so when enough lactose has been metabolized, glucose (one of the products) accumulates and causes repression of the lac operon.
Repressible Operons
Repressible operons are organized in much the same way as inducible operons: there are structural genes under the control of a promoter and operator, and there is a gene encoding a repressor. Repressible operons are regulated not by a reactant in the metabolic pathway (such as lactose was in its metabolic pathway), but by the end product of the pathway. An example of this is the Trp operon, which encodes enzymes responsible for the synthesis of the amino acid tryptophan (trp for short). The trp operon is regulated by trp, which is the product of the metabolic pathway.
Regulation of the trp operon works like this: the trp repressor only binds to the operator when trp is present. (Note: this is the opposite situation to the lac repressor.) The repressor binds to trp, and undergoes a conformational change that allows it to bind to the operator, blocking transcription of the operon. Because trp is needed for repression, it is referred to as a co-repressor in this system (as opposed to lactose being an inducer). When trp is absent, the repressor will not bind to the operator, and transcription of the operon occurs. In this way, if there is plenty of trp around, no more is needed, so the operon is repressed. If there is no trp around, it needs to be synthesized, and the operon is transcribed, allowing the production of the enzymes for trp synthesis.
Consolidate your understanding of repressible operons by sketching out the events described above. Once you have a good grasp of trp operon regulation, figure out the effect of each of the following mutations:
Compare the outcomes in these cases to the analogous mutations in the lac operon.
Prokaryotic Gene Regulation: Summary of Key Concepts