In Lac operon, Catabolite gene activator protein (CAP) is responsible for:
Ans. A. Positive regulationThe lac operona. An operon can be regulated by a single promoter or regulatory region. The cistron is the smallest unit of genetic expression.b. When E coli is exposed to both lactose and glucose as sources of carbon, the organisms first metabolize the glucose and then temporarily stop growing until the genes of the lac operon become induced to provide the ability to metabolize lactose as a usable energy source. Although lactose is present from the beginning of the bacterial growth phase, the cell does not induce those enzymes necessary for catabolism of lactose until the glucose has been exhausted. This phenomenon was first thought to be attributable to repression of the lac operon by some catabolite of glucose; hence, it was termed catabolite repression.c. It is now known that catabolite repression is in fact mediated by a catabolite gene activator protein (CAP) in conjunction with cAMP. This protein is also referred to as the cAMP regulatory protein (CRP). The expression of many inducible enzyme systems or operons in E coli and other prokaryotes is sensitive to catabolite repression.d. The lactose (lac) operon is a classic model for understanding gene regulation. It is negatively controlled through two regulatory genes--the lac I gene that constitutively (always) expresses a repressor protein and the operator (o) region to which the repressor binds. The lac operon is inducible by lactose and lactose analogues, inactivating the repressor and uncovering the operon and its neighboring promoter (p) sequence.e. Several operons in E. coli, including the lac operon, are subject to catabolite repression. In the presence of glucose, there is decreased manufacture of cyclic AMP (cAMP) by adenylate cyclase. Low glucose levels increase production of cAMP, which binds to the catabolite activator protein (CAP). The cAMP-CAP complex binds to the promoters of several responsive operons at catabolite activator protein (CAP) binding sites, greatly enhancing transcription of operon RNA. This positive control stimulates use of more exotic metabolites when glucose is not available and conserves energy when glucose is plentiful. High levels of glucose lower cAMP levels and direct metabolism toward constitutive glucose pathways such as glycolysis.
Positive regulation
Negative regulation
Constrictive expression
Alteration
Molecular Genetics