Question

Question. Discuss the expression of structural genes in lac operon, when activator (CAP binding site) of lac operon is mutated and growth medium is supplemented with only lactose (glucose is unavailable) as a carbon source. Support your discussion with a simple illustration including elements of lac operon.

Answer 1

Gene cluster, promoter and regulatory sequences which function together in regulation are called as an operon.

The CAP binding site is a positive regulatory site that is bound by catabolite activator protein (CAP). When CAP is bound to this site, it promotes transcription by helping RNA polymerase bind to the promoter.

CAP isn't always active (able to bind DNA). Instead, it's regulated by a small molecule called cyclic AMP (cAMP). cAMP is made by E. coli when glucose levels are low. cAMP binds to CAP, changing its shape and making it able to bind DNA and promote transcription. Without cAMP, CAP cannot bind DNA and is inactive.

When Glucose is absent but lactose is present:

Strong transcription of the lac operon occurs. The lac repressor is released from the operator because the inducer (allolactose) is present. cAMP levels are high because glucose is absent, so CAP is active and bound to the DNA. CAP helps RNA polymerase bind to the promoter, permitting high levels of transcription.

But when activator (CAP binding site) of operon is mutated, CAP cannot bind to this site. So RNA polymerase cannot bind to promoter and therefore no transcription will occur even if growth medium is supplemented only with lactose and glucose is absent.

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