How is glucose involved in catabolite repression of lactose degradation?
What will be an ideal response?
In E. coli, the sugar lactose induces transcription of genes that encode beta-galactosidase (lacZ) and lactose permease (lacY). But in the presence of glucose, a preferred carbon source, lac transcription is halted. Halting lac transcription enables preferential catabolism of glucose. Substrates are selected through gene regulation.
lacZ and lacY are part of an operon in E. coli called the lac operon. The operon is inducible by lactose to the maximal levels when cAMP (cyclic adenosine monophosphate) and CAP (catabolite activator protein, an activator protein) form a complex.
(a) Under conditions of high glucose, a glucose breakdown product inhibits the enzyme adenylate cyclase, preventing the conversion of ATP into cAMP.
(b) Under conditions of low glucose, there is no breakdown product, and therefore adenylate cyclase is active and cAMP is formed.
(c) When cAMP is present, it acts as an allosteric effector (and in inducer), complexing with CAP.
(d) The cAMP-CAP complex acts as an activator of lac operon transcription by binding to a region within the lac promoter.
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