regulation of gene expression

Regulation of Gene expression

Learning Goals:To know and explain: Constitutive ( house keeping) vs. Controllable genesOPERON 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

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.

Gene Regulation

Control MechanismsOver 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

Gene regulationGene expression/ regulation can occur at 4 levels:

1. Transcriptional2. Post-transcriptional3. Translational 4. Post-translational

A promoter is a region of DNA that attracts the RNA polymerase complex to bind and begin transcription.

Regulation in prokaryotes

Regulation is required when switching metabolic pathway dependent upon what nutrients are available

3 levels of regulation:1. During transcription2. During translation 3. After protein synthesis

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.

Gene regulation

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 2- 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.

General structure of an 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.

The lac Operon

Figure 8.13

No repressor

With repressor

The activity of an Operon in the presence or the absence of repressor

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

Gene Regulation in Eukaryotes

Regulation is much more complex:

• Pre-transcriptional• Transcriptional• Post-transcriptional• Translational• Post translational

Gene Regulation in Eukaryotes

– 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.

1- Control at DNA level by DNA methylation

• Acetylation by HATs and coactivators leads to euchromatin formation

• Methylation by HDACs and corepressors leads to heterochromatin formation

2- Control at DNA level by Histone modifications(Chromatin

Remodeling)

3-Control at DNA level by gene amplification

Repeated rounds of DNA replication yield multiple copies of a particular chromosomal region.

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 control region for gene X

gene X

promoter

Calcitonin gene-related peptide

61

5- Control at mRNA splicing (alternate splicing)

cell

1

cell

2

(four exons)1 2 3 4

1, 2 & 3 1, 2 & 4

32 amino acidsReduces bone resorption

37 amino acidsVasodilator

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

7- Control at initiation of translation

5’ UTR 3’ UTRAUG UAA

Specific sequences make specific secondary structuresSpecific protein factors bind to these secondary structures

8-Regulation by protein stability

• 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 minN-terminal : For example methionine, alanine, : t1/2 >20 hrs.

COOH+NH2

NH2

ATP

CO NHCO NH

ubiquitin protein ligase

Doomed protein molecule

26S proteasom

e

•Ubiquitin-dependent proteolysis. Cyclins control of cell cycle.• Protein molecule is tagged for degradation by attachment of a 20 kDa protein, ubiquitin

Regulation by water soluble Hormones

Regulation by lipid soluble HormonesSteroid hormones pass through the cell membrane

and bind cytoplasmic receptors, which togetherbind directly to DNA and regulate gene expression.

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