Question

Discuss, in detail, catabolite repression in the Lac operon. Indicate how this system is regulated
under various nutrient scenarios, all relevant components involved, and how these components
interact.

Answer 1

Catabolite repression is a type of positive control of transcription in which the regulatory protein upregulates the transcription of an operon. The microorganism utilizes and metabolize the preferred carbon and energy source by inhibiting the synthesis of enzymes involved in catabolism of other carbon sources. The lactose or lac operon is active in presence of lactose in the medium and is required for the transport and metabolism of lactose in E. coli and other enteric bacteria when glucose is not available. The structure of lac operon consists of a regulator (Lac I), a promoter (P), an operator (O), three structural genes (Lac Z, Lac Y and Lac A) and a terminator. Transcription of all genes starts with the binding of RNA polymerase enzyme on the promoter region. When glucose and lactose bothe are present as carbon and energy source, the bacterial cell will first utilize glucose as energy source then follwed by lactose. When glucose is present in the medium, the cell will not metabolise lactose. Regulation of lac operon takes place through various mechanism as shown in the figure. The first mechanism involves regulatory or repressor protein synthesized by regulator region situated to the upstream of promoter region. In absene of lactose, this repressor protein binds tightly to operator region blocking it for binding of RNA polymerase enzyme. Thus the transcription of Lac operon is inhibited. In presence of lactose, lactose acts as inducer binding to repressor protein making it inactive. So the inactive repressor protein is unable to bind to operator and leaves operator region. As a result, this site becomes available for RNA polymerase binding and transcription takes place. Allolactose also acts as inducer for Lac operon. The second mechanism is response to glucose utilizing catabolite activator protein (CAP) homodimer. Cyclic adenosine monohosphate (cAMP) acts as a signal molecule which activates CAP. In absence of glucose, cyclic adenosine monohosphate (cAMP) concentration increases, which then binds to CAP and recruits it to CAP binding site upstream of promoter region. Binding of cAMP+CAP to the CAP binding site initiates the transcription of Lac operon and thus metabolizes lactose for generation of energy.