7. (4 pts) During an internship at the National Institute of Health, Rupert is a
ID: 252697 • Letter: 7
Question
7. (4 pts) During an internship at the National Institute of Health, Rupert is asked to examine regulation of the lac operon of E. coli under 4 different conditions. Rupert has forgotten what he learned in Genetics lab, so has called you for help. Using the chart below, determine if Rupert should expect: a. the repressor protein () to be bound to the operator preventing expression b. the cAMP to be high or low c. lacZ to be expressed d. level of lacZ expression Repressor CAMP leve Lacz Bound (YN) (High/Low expressed (High/Low/None) LacZ level (YN) -Glucose -Lactose -Glucose +Lactose +Glucose +Lactose +Glucose LactoseExplanation / Answer
Some catabolic breakdown product of glucose prevents activation of the lac operon by lactose, so this effect was originally called catabolite repression. When glucose is present in high concentrations, the cAMP concentration is low; as the glucose concentration decreases, the concentration of cAMP increases correspondingly. The high concentration of cAMP is necessary for activation of the lac operon.
The catabolite activator protein (CAP), when bound to cAMP, is a positive regulator of the lac operon. CAP forms a complex with cAMP which binds to the CAP site of the operon. The DNA-bound CAP is then able to interact physically with RNA polymerase and essentially increase the affinity of RNA polymerase for the lac promoter leading to the selective activation of the lac operon.
a) No glucose, no lactose- Lac repressor binds to the operator. No lacZ.
b) No glucose, lactose present - cAMP levels will be high, so CAP protein and cAMP will bind to the lac operon to enhance transcription. In the presence of lactose, the lac repressor will not bind to the lac operator, allowing transcription. The presence of CAP-cAMP will cause abundant transcription.
c) Both glucose and lactose present- cAMP levels will be low, no CAP protein binding to the lac operon to enhance transcription. In the presence of lactose, the lac repressor will not bind to the lac operator, allowing a low level of transcription.
d) Glucose present, no lactose- cAMP levels will be low, so CAP protein will not bind to the lac operon to enhance transcription. In the absence of lactose, the lacrepressor will remain bound to the lac operator, preventing transcription.
Repressor bound
(Y/N)
cAMP level
(High/Low)
LacZ expressed (Y/N)
LacZ level (High/Low/None)
-Glucose
-Lactose
Y
High
N
None
-Glucose
+Lactose
N
High
Y
High
+Glucose
+Lactose
N
Low
Y
Low
+Glucose
-Lactose
Y
Low
N
None
Repressor bound
(Y/N)
cAMP level
(High/Low)
LacZ expressed (Y/N)
LacZ level (High/Low/None)
-Glucose
-Lactose
Y
High
N
None
-Glucose
+Lactose
N
High
Y
High
+Glucose
+Lactose
N
Low
Y
Low
+Glucose
-Lactose
Y
Low
N
None
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