E. coli cells are growing in a medium containing lactose but no glucose. Indicat
ID: 226406 • Letter: E
Question
E. coli cells are growing in a medium containing lactose but no glucose. Indicate whether each of the following changes or conditions would increase, decrease, or not change expression of the lac operon a. Addition of a high concentration of glucose b. A mutation that prevents Lac repressor binding to the operator c. A mutation that completely inactivates (1-galactosidase d. A mutation that completely inactivates galactoside permease e. A mutation that prevents binding of CRP to its binding site near the Lac promoterExplanation / Answer
1. a. Addition of a high concentration of glucose to the medium would decrease the expression of lac operon. It is because increased glucose levels, decrease the levels of cAMP (cyclic adenosine monophosphate). cAMP binds with catabolite activator protein (CAP, also known as cAMP receptor protein (CRP)) and this complex assists the binding of RNA polymerase to the promoter. As a result, in presence of glucose, transcription of lac operon is reduced. This process is known as catabolite repression.
b. A mutation that prevents lac repressor binding to the operator would not change the expression of lac operon in presence of lactose. Lac repressor binding to its operator results in the turning off of lac operon in the absence of lactose. As, the cells are being grown in a medium containing lactose (inducer), the operon is already turned on. So, mutation in the lac repressor would not change the expression of the operon in presence of lactose.
c. A mutation that completely inactivates beta-galactosidase enzyme will not change the expression of lac operon. It is because the enzyme converts lactose to glucose and galactose. As, glucose, is not formed there will not be catabolite repression of the operon and so the operon remains turned on in presence of lactose, Further, since there is no mutation in the promoter or operator region, the operon remains turned on in presence of lactose.
d. A mutation that completely inactivates galactoside permease will decrease the expression of lac operon. It is because galactoside permease converts lactose to allolactose and this allolactose binds to the repressor. Binding of allolactose to the repressor will remove the regulation on lac operator region. As a results the RNA polymerase now can bind to the promoter region and transcribe the lac operon. Since, because of mutation in the galactoside permease gene, repressor remains bound to the operator region and there will not be transcription of the operon.
e. A mutation that prevents binding of CRP to its binding site near the lac promoter will decrease the expression of lac promoter. It is because cAMP-CRP complex is required for the binding of RNA polymerase to the promoter region. Since CRP gene is mutated, cAMP-CRP complex cannot now bind to the binding site near the lac promoter, as a result RNA polymerase is not recruited to the promoter region and so there will be no transcription of the lac operon.
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