14th Lecture : 14/11/14

Topic: Gene regulation

Notes:
1) There are two modes of gene expression :

• Constitutive expression: Gene is expressed at the same rate all the times
• Inducible expression: Gene expression changes in response to condition

2) There are two major modes of regulation in cell:

 - control the activity of preexisting enzyme

• Post-transcriptional or post-translational - the synthesis of product can be prevented by inhibit the activity of enzyme

 - control the amount of enzyme synthesized

• Transcriptional regulation - the synthesis of product can be prevented by control the level of transcription
• Translation regulation - the synthesis of product can be prevented by control the level of translation

3) The enzyme activity can be regulated by feedback inhibition. Feefback inhibition involves many steps and the end products of the pathway inhibits the activity of the first enzyme. When the end product is used up, the synthesis of product can be resume.

4) Allosteric inhibition is responsible for the end product inhibition because allosteric enzyme has 2 binding sites : active sites and allosteric sites. When the inhibitor(end product) binds to the allosteric sites, the conformation of enzyme changes and substrate no longer binds to the active site. When the concentration of inhibitor(end product) falls, inhibitor dissociates from allosteric site and the active site restores shape.

5) Most bacterial regulation occurs at the transcription level because it would be a waste to make the RNA if neither the RNA notr its encoded protein is needed.

6) There are two types of regulation of transcription: Repression and Induction.

7) To turn "on" or "off" of gene, regulatory protein and environmental sensor are needed. Regulatory protein produces activator or repressor and environmental sensor is small moleccule that activates or inactivates regulatory protein.

8) Enzyme repression inhibit the gene expression and decrease the synthesis of enzyme and it is response to abundance of end-product. Transcription of the operon occur because the repressor is not bind to the operator. When the corepressor binds to the repressor and the repressor binds to the operator, the transciption is blocked. mRNA and proteins it encodes are not made.

9) Enzyme induction turns on the transciption of genes and synthesise the enzyme only when its substrate is present. A repressor binds to the operator and blocks the transciption of the genes. An inducer molecule binds to the repressor and inactivate it. Transcription by RNA polymerase occurs and an mRNA for that operon is formed.

10) Description of trp operon:

trpE, trpD, trpC, trpB, trpA are the structural genes for the tryptophan synthesis. The end product for trp operon is tryptophan. The regulatory gene, trp R will produces mRNA and translate to inactive repressor. When tryptophan is absent, the repressor is inactive and the operon is on. RNA polymerase attached to the promoter and trancribes the operon's genes. When tryptophan is present, it act as corepressor and binds to the repressor to activate the repressor. The operon is off. The activated repressor binds to the operator and blocks the transcription of the operon;s genes. The end product tryptophan itself act as corepressor. Therefore, it is a feedback inhibition which the end product inhibits the activity of the first enzyme.

11) Description of lac operon:

lac Z, lac Y and lac A are the structural genes for lactose synthesis. Lactose are not the preferred carbohydrate source for E.coli. There are CAP site, promoter site and operator site for lac operon. When glucose is present, the lac operon is off even lactose is present because E.coli prefer used glucose as first carbohydrate source. The cAMP level is low and the binding of CAP-cAMP complex to the CAP site is acheived and the RNA-polymerase able to dissociate with the promoter but the active repressor binds to the operator causes the inhibition of transcription. If glucose is absent, the operon is on. The concentration of cAMP increased. The repressor protein binds to allolactose and incapable interact with the operator. CAP-cAMP complex binds to the CAP site and help in the binding of RNA polymerase to the promoter, structural genes are able to transcript and lactose is synthesised. Nevertheless, if glucose is present, the lac operon is repressed even in the presence of lactose. This repression is maintained until the glucose supply is exhausted.

Activities: No other activity carried out in class except for teaching...^^

My own explorace:

1) Too much of tryptophan exist in bacteria is toxic. Therefore, they need to regulate the amount of tryptophan.

2) Cyclic adenosine monophosphate is a second messenger important in many biological processes. cAMP is derived from adenosine triphosphate (ATP) and used for intracellular signal transduction in many different organisms.

Reflection:

I learnt gene regulation in Form 6 but I learnt new things in this lecture, the cAMP and CAP site. We didn't touch about the cAMP and CAP site for gene regulation chapter in Form 6. The lac operon consists of CAP site and it is for the binding of complex cAMP-CAP. This complex will enhances the dissociation of RNA polymerase to promoter. Everybody felt tired after came back from NUCELL and someone felt sleepy in this lecture but I still concentrate in this lecture because genetic is an interesting topic for me.