What is the purpose of the lac operon, and under what circumstances is it activated? Describe the structure of the lac operon, including the repressor protein, structural genes, and regulatory regions of the operon

Question

What is the role of catabolite repression in the lac operon?

Answer 1

The lac operon is responsible for the production of three polypeptides that permit E. coli to use the sugar lactose as a carbon source for growth and metabolic energy.
Glucose is the preferred energy source of E. coli, but lactose can serve as an alternative carbon source for bacteria. The lac operon acts as a genetic switch to begin breaking down lactose, but only if (1 ) lactose is present in the cell; and (2 ) all glucose has been depleted.
The lac operon consists of a multipart regulatory region and a structural gene region containing three genes. The regulatory region contains three protein-binding regulatory sequences: the promoter that binds RNA polymerase, the operator (lacO) that binds the lac repressor protein, and the CAP-cAMP region.
The three structural genes of the lac operon are lacZ, a gene encoding the enzyme β-galactosidase; lacY, which encodes the enzyme permease; and lacA, which encodes transacetylase. The β-galactosidase enzyme cleaves the β-galactoside linkage of lactose to yield glucose and galactose. The enzyme also converts lactose into allolactose, which acts as the inducer for the operon. The permease enzyme product of the lacY gene controls entry of lactose into the cell. Transacetylase,
the product of lacA, is not essential for lactose utilization, although in bacteria it protects against potentially damaging by-products of lactose metabolism. The regulatory gene, lacI, produces the lac repressor and has its own promoter, which is not regulated and drives constitutive transcription. The lac operon promoter, lacP, contains the consensus sequence sites that are critical for RNA polymerase binding. LacO, which binds the repressor protein produced by lacI, overlaps lacP near the start of transcription.
The removal of the lac repressor by allolactose alone will not result in maximal induction of the lac operon. The binding of the CAP-cAMP complex to the CAP binding region of the promoter is also required. cAMP is not normally found in high concentrations in the presence of abundant glucose, because glucose inhibits adenylate cyclase, the enzyme that converts ATP to cAMP. Thus, the preferred catabolite, glucose, helps to prevent expression of the lac operon genes, a process called "catabolite repression." In the absence of glucose, however, cAMP levels rise, allowing this molecule to bind to the catabolite activator protein (CAP). This complex binds to the lac promoter at the CAP binding site, bending the DNA to allow increased transcription by RNA polymerase.