Final answer:
The lac operon in E. coli is regulated by two main mechanisms: inducer exclusion and multiple operators. When glucose is present in high levels, it prevents the expression of the operon through catabolite repression. When lactose is present and glucose is limited, allolactose acts as an inducer and allows for the transcription of the structural genes.
Step-by-step explanation:
The lac operon is a regulatory system in E. coli bacteria that controls the expression of genes involved in lactose metabolism. It consists of a promoter, an operator, and three structural genes (lacZ, lacY, and lacA). The lac operon is regulated by two main mechanisms: inducer exclusion and multiple operators.
When glucose is present in high levels, it inhibits the expression of the lac operon through a process called catabolite repression. Glucose prevents the formation of cyclic AMP (cAMP), which is necessary for the catabolite activator protein (CAP) to bind to the operator and activate transcription of the operon.
When lactose is present and glucose is limited, lactose is converted to allolactose. Allolactose acts as an inducer and binds to the lac repressor, preventing it from binding to the operator. This allows RNA polymerase to bind to the promoter and initiate transcription of the structural genes.