1. Regulation of Gene expression

2. Gene Regulation Learning Goals: To know and explain:
Constitutive ( house keeping) vs. Controllable genes
OPERON structure and its role in gene regulation
Regulation of Eukaryotic Gene Expression at different levels:
DNA methylation
Histone modifications(Chromatin Remodeling)
Increasing the number of gene copies (gene amplification)
Changing the rate of initiation of transcription
Alternate splicing
mRNA stability
Changing the rate of initiation of translation
Protein stability
Hormonal regulation
11-Regulation by protein stability

3. Gene Regulation What makes one cell different from another cell?
Regulation not only involves which genes are active or inactive, but also determines the level of activity and the amount of protein that is available in a cell.
Genes which are always active are called constitutive genes, or housekeeping genes.

4. Control Mechanisms
Over 20,000 human genes – but not all are needed as proteins at the same time
It would be energy inefficient to synthesize all of them all the time!
Thus, gene regulation:
The turning on or off of specific genes as required by an organism
But there are still housekeeping genes:
Genes that are continually transcribed and translated because they are vital to the organism’s life functions
And to help, there are transcription factors:
Proteins that bind to DNA that needs to be transcribed to help RNA polymerase bind and transcribe more easily

5. Gene regulation
Gene expression/ regulation can occur at 4 levels:
Transcriptional
Post-transcriptional
Translational
Post-translational
A promoter is a region of DNA that attracts the RNA polymerase complex to bind and begin transcription.

6. Gene regulation
Regulation in prokaryotes
Regulation is required when switching metabolic pathway dependent upon what nutrients are available
3 levels of regulation:
During transcription
During translation
After protein synthesis

7. Gene regulation
OPERON:
In prokaryotes, an area that includes a promoter and many genes clustered together under control of a single promoter
Genes involved in the same metabolic pathway are often found in the same operon.
Genes can be expressed in a single mRNA strand called polycistronic mRNA.
**Polycistronic – multiple gene products on the same piece of DNA.

8. OPERON in gene regulation of prokaryotes
Definition: a few genes that are controlled collectively by one promoter
Its structure: Each Operon is consisted of few structural genes( cistrons) and some cis-acting element such as promoter (P) and operator (O).
Its regulation: There are one or more regulatory gene outside of the Operon that produce trans-acting factors such as repressor or activators.
Classification:
1- Catabolic (inducible) such as Lac OPERON Anabolic (repressible) such as ara OPERON
3- Other types
An operator is a DNA segment that turns the gene "on" or "off." It interacts with proteins that increase the rate of transcription or block transcription from occurring.

9. General structure of an OPERON

10. The lac Operon
E.Coli can adjust gene expression according to the sugar is available.
Genes that encode the enzymes for are needed to break down lactose are found in the lac operon. The lac operon consists of a coding region and a regulatory region
Also contains:
Promoter
Operator- a DNA sequence to which a protein binds to inhibit transcription initiation (repressor)
The CAP is a DNA sequence to which a specific protein also binds. The binding of CAP increases the rate of transcription of a gene or genes.

11. The activity of an Operon in the presence or the absence of repressor
No repressor
With repressor
Figure 8.13

14. To summarize… Lac Operon Inducible operon
Repressor (LacI) is usually ON the operator
Repressor changes shape and falls off when the inducer (lactose) binds to the repressor, allowing transcription
Trp Operon
Repressible operon
Repressor is usually OFF the operator
Co-repressor (tryptophan) changes the repressor’s shape so it can attach to the operator, blocking transcription

15. Gene Regulation in Eukaryotes

16. Gene Regulation in Eukaryotes
Regulation is much more complex:
Pre-transcriptional
Transcriptional
Post-transcriptional
Translational
Post translational

17. 1- Control at DNA level by DNA methylation
Heterochromatin is the most tightly packaged form of DNA. transcriptionally silent, different from cell to cell
Methylation is related to the Heterochromatin formation
Small percentages of newly synthesized DNAs (~3% in mammals) are chemically modified by methylation.
Methylation occurs most often in symmetrical CG sequences.
Transcriptionally active genes possess significantly lower levels of methylated DNA than inactive genes.
Methylation results in a human disease called fragile X syndrome; FMR-1 gene is silenced by methylation.

18. 2- Control at DNA level by Histone modifications(Chromatin Remodeling)
Acetylation by HATs and coactivators leads to euchromatin formation
Methylation by HDACs and corepressors leads to heterochromatin formation

19. 3-Control at DNA level by gene amplification
Repeated rounds of DNA replication yield multiple
copies of a particular chromosomal region.

20. 4- Control at transcription initiation
By using different sequences (promoter, enhancer or silencer sequences) and factors, the rate of transcription of a gene is controlled
gene X
promoter
gene control region for gene X

21. 5- Control at mRNA splicing (alternate splicing)
(four exons) 1 2 3 4
1, 2 & 4
1, 2 & 3
cell 2
cell 1
Calcitonin gene-related peptide 61
32 amino acids
Reduces bone resorption
37 amino acids
Vasodilator

22. 6- Control at mRNA stability
Some hormones which enhance the production of proteins also increase the half life of the protein’s mRNA.
Estrogen : ovalbumin t1/2 from 2- 5hr to >24hr
Prolactin : casein t1/2 from 5 hr to 92hr
mRNA stability. When milk protein synthesis is stimulated in the mammary epithelium at child birth, the rapid increase in casein level arising from the pituitary hormone prolactin results from increased transcription of the casein gene but also from stabilisation of its mRNA. In fact, the stabilisation of mRNA is an essential component of the rapid build up of casein protein and this sort of regulation is evident in many situations in which the production of a particular protein needs to be increased to a high level.
The mechanism is not well understood. The poly A tail protects the mRA from 3’ degradation. Histone mRNA (histones are produced during the DNA synthetic phase of the cell cycle) do not have a poly(A) tail and are unstable. The sequence of bases in the 3’ untranslated region, especially runs of As and Us can affect the stability of individual mRNAs. However, the enzzymes that break down the RNA are not well characterised.

23. 7- Control at initiation of translation
5’ UTR
3’ UTR
AUG
UAA
Specific sequences make specific secondary structures
Specific protein factors bind to these secondary structures

24. 8-Regulation by protein stability
Ubiquitin-dependent proteolysis. Cyclins control of cell cycle.
Protein molecule is tagged for degradation by attachment of a 20 kDa protein, ubiquitin
ATP CO NH
NH2 +
COOH CO NH
26S proteasome
Doomed protein molecule
ubiquitin protein ligase
The stability of a protein depends upon its N-terminal amino acid (the N-end rule).
N-terminal : For example arginine , lysine : protein t1/2 = 3 min
N-terminal : For example methionine, alanine, : t1/2 >20 hrs.

25. Regulation by water soluble Hormones

26. Regulation by lipid soluble Hormones
Steroid hormones pass through the cell membrane
and bind cytoplasmic receptors, which together
bind directly to DNA and regulate gene expression.

28. Insert your summary here.......