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  #1  
Old 04-30-2012
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Bacteria E. coli Lactose Metabolism

self-made, beware of mistakes.
===================

In a media, containing lactose, but no glucose, e.coli bacteria is expressing lac Z, Y and A genes, B-galactosidase is expressed and lactose is being hydrolized to glucose. What is the key event that allows this enzyme to be expressed?

a. low levels of glucose.
b. Lac repressor bound to lac operator.
c. Lac repressor bound to promoter.
d. low cAMP levels.
e. high cAMP levels.
f. CAP bound to lac repressor.

====================
just for the sake of fun
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  #2  
Old 04-30-2012
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Glucose inhibits the rise in cAMP which allows the transcription of the gene, so in case we are lack of glucose cAMP levels will rise activate the activator which will transcribe the gene.
So it should be E
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I think E....plz correct
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Old 04-30-2012
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Quote:
Originally Posted by kdm0101 View Post
I think E....plz correct
second that , but
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  #5  
Old 04-30-2012
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Default great question!

@ DocSikorski - I think it's a really good question! Especially for those who are weak in micro - like me...

I think it's E.

my explanation:

In E. coli lactase is only synthesized in the presence of lactose.
(coli lives in the GI, so it must live on whatever the host eats

- Genes encoding the lactase have operators and promotors
- Without any lactose present, lac repressor binds to the operator site
- When lactose is present this repressor is removed
- Once it's removed, RNA polymerase binds to the promoter and RNA is transcribed -> lactase

about cAMP I have no idea what does that have to do with lactase synthesis - I thought that the presence of lactose itself turns on the synthesis. Is it just a distractor?

I'll think about this question next time I grab a glass of milk
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Old 05-01-2012
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Quote:
Originally Posted by Casandra View Post
@ DocSikorski - I think it's a really good question! Especially for those who are weak in micro - like me...

I think it's E.

my explanation:

In E. coli lactase is only synthesized in the presence of lactose.
(coli lives in the GI, so it must live on whatever the host eats

- Genes encoding the lactase have operators and promotors
- Without any lactose present, lac repressor binds to the operator site
- When lactose is present this repressor is removed
- Once it's removed, RNA polymerase binds to the promoter and RNA is transcribed -> lactase

about cAMP I have no idea what does that have to do with lactase synthesis - I thought that the presence of lactose itself turns on the synthesis. Is it just a distractor?

I'll think about this question next time I grab a glass of milk
CAMP induces catabolite activated protein(CAP) which further helps RNA Poly bid to Promoter. CAMP is inhibited by Glucose, so that's why for Lac operon, presence of Lactose and Absence of Glucose is required. Hope I am right for a change.
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Old 05-01-2012
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the answer is D:

low cAMP levels:

lac repressor is not bound to lac operon,
CAP is bound to CAP-binding site due to low cAMP levels: B-galactosidase (not lactase!) is transcribed and lactose is hydrolized to glucose.
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I think CAP binds to activator protein when cAMP level is High not Low. It happens when Glucose is absent which otherwise decreases cAMP LEVEL. Reference to Kaplan n FA 2012 page 76.
Quote:
Originally Posted by DocSikorski View Post
the answer is D:

low cAMP levels:

lac repressor is not bound to lac operon,
CAP is bound to CAP-binding site due to low cAMP levels: B-galactosidase (not lactase!) is transcribed and lactose is hydrolized to glucose.
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  #9  
Old 05-01-2012
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Quote:
Originally Posted by kdm0101 View Post
I think CAP binds to activator protein when cAMP level is High not Low. It happens when Glucose is absent which otherwise decreases cAMP LEVEL. Reference to Kaplan n FA 2012 page 76.
CAP (catabolite activator protein; Inducer)
is a gene regulatory protein, that binds to cis-acting DNA sequence (CAP-binding site) upstream of the lac promoter when cAMP levels are low and increases the transcription of lac Z, lac Y and lac A genes of the lac operon.

-- given by HY Molecular biology.
=======================

Wikipedia: Cyclic adenosine monophosphate (cAMP) is a signal molecule whose prevalence is inversely proportional to that of glucose. It binds to the CAP, which in turn allows the CAP to bind to the CAP binding site (a 16 bp DNA sequence upstream of the promoter on the left in the diagram below), which assists the RNAP in binding to the DNA. In the absence of glucose, the cAMP concentration is high and binding of CAP-cAMP to the DNA significantly increases the production of β-galactosidase, enabling the cell to hydrolyse (digest) lactose and release galactose and glucose.

=======================

Ok, this must be a mistake in some of these resources.
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Last edited by DocSikorski; 05-01-2012 at 08:15 AM.
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Old 05-01-2012
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http://www.ncbi.nlm.nih.gov/pmc/arti...8/?tool=pubmed

the answer has to be E. high cAMP.

sorry for the confusion, blame Ronald W. Dudek
==============================
all i can say is: watch out for mistakes....
http://rumorsweretrue.wordpress.com/...cular-biology/

i am writing an email to this author, lets see what happens
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Last edited by DocSikorski; 05-01-2012 at 08:38 AM.
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  #11  
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Ok, so response from Dr.Dudek was:

high levels of cAMP.
he said that the 3rd edition of the book does not have this mistake.

I have 2nd edition.
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Old 05-17-2012
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[QUOTE=DocSikorski;115028]self-made, beware of mistakes.
===================

In a media, containing lactose, but no glucose, e.coli bacteria is expressing lac Z, Y and A genes, B-galactosidase is expressed and lactose is being hydrolized to glucose. What is the key event that allows this enzyme to be expressed?

a. low levels of glucose.
b. Lac repressor bound to lac operator.
c.Lac repressor bound to promoter.
d. low cAMP levels.
e. high cAMP levels.
f. CAP bound to lac repressor.


This part in USMLE always is my weak I will try to keep on moving on it, so thank you for this post
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Old 08-18-2012
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Just a little note,

For E.coli RNA polymerase to be able to transcribe the lac operon, two signals must be available;

1- cAMP and CAP are bound to CAP binding site.
2- allolactose is bound and inhibiting the repressor from attaching the operator .

If there is lactose and no glucose , there is gene expression.
If there is glucose and no lactose , there is no gene expression, and e coli is feeding on the glucose .
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  #14  
Old 08-18-2012
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this stuff was on my test, thank God I understood it in this thread...
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