25. Describe how the availability of lactose and glucose control transcription i

Question

25. Describe how the availability of lactose and glucose control transcription in a bacterial cell. You may use diagrams to assist in your explanation a. (4 points) Describe the organization and function of the lactose operon. Describe what happens when lactose is absent and what happens when lactose is present (glucose is absent in both of these cases, and you don't need to consider the effects of glucose for this answer). Be detailed in your explanation. b. (4 points) Describe how catabolite repression functions in control of the lactose operon. Describe what happens when glucose is present and what happens when glucose is absent (you don't need to consider lactose availability for this answer). Be detailed in your explanation.

Explanation / Answer

B) When glucose is available, it is used in preference as carbon source. So when E. coli finds glucose in the medium, it metabolizes and represses the use of other sugars. The ability of glucose to inhibit the synthesis of certain enzymes is referred as the glucose effect, resulting in diauxic growth. This phenomenon is due to the repressive effect of glucose on the synthesis of enzymes required for metabolism of other sugars.

Catabolite repression is caused by a reduction of the level of intracellular cAMP. CRP is a positive regulator protein activated by cAMP, needed for RNA polymerase to initiate transcription of operons. So when glucose level in the cell is high, the cAMP concentration is low, that turns OFF the transcription

When the glucose level is low, the cAMP concentration is high. CRP-cAMP bind together to form a complex and binds to the promoter region in order for transcription of the lac operon to occur

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