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#51 Biochemistry Gene Expression III Lecture for Kevin Ahern's BB 451/551

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Published on Mar 12, 2012

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1. Enhancer sequence elements allow transcription factors that a made in specific tissues to specifically activate (or in some cases inhibit) transcription of genes under their control in those specific tissues.

2. Modifications to the chromatin (by acetylation/deactylation of lysines in histones) can activate or inactivate (respectively) transcription of genes in eukaryotes. Alterations to the DNA, such as methylation of cytosine can act to inactivate transcription.

3. Nuclear hormone receptors, such as the estrogen receptor, have DNA binding domains and ligand binding domains. The binding of the estradiol (and estrogen) ligand to the estrogen receptor causes a conformational change in the protein, but does not change the binding of the protein to DNA. Binding of the estradiol DOES appear to activate the protein and thus activate transcription of the genes that the receptor binds to the promoter of.

4. The key to action of the nuclear hormone receptor that binds estradiol is that binding of estradiol favors binding of the receptor to co-activator proteins. These co-activator proteins help to turn on transcription of the relevant genes

5. An antagonist of the estrogen receptor is the drug tamoxifen. Binding of tamoxifen by the estrogen receptor stops the receptor from activating transcription of genes that it normally activates.

6. Tamoxifen appears to act by binding the estrogen receptor (I use the terms estrogen receptor and nuclear hormone receptor here as the same thing), with a part of the molecule extending into the region of the protein that normally binds to co-activators. Thus, tamoxifen acts by stopping recruitment by the receptor of co-activators.

7. Altering chromatin structure is an essential function for transcriptional activation in eukaryotes. Co-activator proteins appear to play a role in this process by catalyzing the acetylation of lysine residues in histones.

8. Proteins involved in transcriptional control often have bromodomains. These regions of protein recognize and bind to acetylated lysine residues in histones.

9. Altering chromatin structure involves a process called remodeling. Steps in this process include 1) binding of a transcription factor to a promoter sequence; 2) recruitment of co-activators; 3) acetylation of histone lysines by co-activators; 4) binding of the 'remodeling engine' at the acetylation site; 5) exposing of DNA by the remodeling engine; and 6) binding of RNA polymerase II to the exposed DNA.

10. In eukaryotic cells, the ferritin mRNA has a region of it called an iron response element that can be bound by a protein called IRP (iron response element binding protein). IRP binds the iron response element when iron is absent. If IRP is NOT bound to the iron response element (high iron conditions), ferritin is made because the IRP does not block the ribosome from translating the mRNA. Thus, when iron concentration is high, ferritin is synthesized to hold it. When IRP is bound to the iron response element (low iron conditions), ferritin is not made. Thus when iron is not available, ferritin is not made.
11. The transferrin receptor has multiple iron response elements at the 3' end of its mRNA. When IRP binds to it, the 3' end is protected and transferrin receptor is made. Thus, when iron is low, the IRE-BP binds the mRNA, protecting it, and the transferrin receptor is made to bring iron into the cells. When iron is high, the IRE-BP leaves the mRNA's 3' end, leaving it susceptible to degradation.

12. Thus when iron inside the cell is high, ferritin is made to hold onto it and when iron is low inside the cell, transferrin receptor is made to bring more iron in.

13. Micro RNAs are short RNA molecules that play an important role in regulating levels of eukaryotic mRNAs. They are complementary to the mRNA the regulate and when combined with the target mRNA in a protein called Argonaute, result in cutting of the mRNA and reducing or eliminating its effectiveness.

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