Question 3: The lac operon - backwards. Under what glucose and lactose condition

Question

Question 3: The lac operon - backwards. Under what glucose and lactose conditions would the lac operon be most highly expressed if the CAP transcriptional activator bound allolactose and the Lac Repressor bound cAMP. Explain your thinking Assume that (i) CAP still activates transcription (ii) Lac Repressor still represses transcription (ii) Allolactose binding to CAP has the same effect on CAP DNA-binding as cAMP binding to CAP (iv) cAMP binding to Lac Repressor has the same effect on Lac Repressor DNA-binding as allolactose binding to Lac Repressor.

Explanation / Answer

Answer: Condition: Low glucose and high lactose

Reason: When the glucose levels are low in the system, it would not be able to repress the system on the operator and hence the RNA polymerase would still be active enough to bring about transcription. On the other hand, if lactose is present in large concentrations in the system, it will generate a positive regulatory force for expression of the lac genes since the homeostatic system would require large amount of lactase enzyme for its degradation. Thus, considering all other conditions favour gene expression, low glucose and high lactose content favour up-regulation of gene expression in the lac operon.

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