Gene expression

11/23/2014

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GENE EXPRESSION

Department of Biochemistry, Kathmandu Medical College Sunday, November

23, 2014

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Gene expression

Sunday, November 23, 2014

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Production of RNA and/or Protein:

Gene expression.

Not all gene are regulated.

Housekeeping genes: genes that are

continuously expressed.

Regulated vs Unregulated gene

expression

Regulatory sequences and molecules


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Regulation controlled by

regulatory sequence

embedded in the non-coding

region of DNA.

Cis-acting element vs Trans-

acting element.

Polycistronic vs Monocistronic


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mRNA carries genetic information from Nucleus to Cytosol.

If mRNA carries information from more than one gene it is said

to be Polycistronic.

If mRNA carries information from just one gene it is said to be

Monocistronic.

Polycistronic is the characteristics feature of Prokaryotes.

Regulation of prokaryotic gene

expression


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Gene expression occurs at transcriptional level

Reason: Energy is not lost making unnecessary product.

Operon: Genes are turned “ON” or “OFF” as a unit. Entire package is called OPERON.

1. Transcription of mRNA from bacterial operons.

2. Role of operator in prokaryotic transcription.

3. The lactose operon.

Lac Operon


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Consists of:

3 structural gene: LacZ, LacY, LacA

Promoter

Terminator

Regulator (Regulatory region)

Operator

Inducer molecules has a role in controlling gene

expression. How?

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The lactose operon


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Lac operon codes for 3 proteins involved in

catabolism of the disaccharide (lactose).

LacZ gene: codes for galactosidase

LacY gene: codes for permease

LacA gene: codes for thiogalactoside transacetylase.

Regulatory portion contains: Promoter (P), Operator

(O) site and CAP / CRP site.

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Role of operator in prokaryotic

transcription (Lactose operon of E.coli)


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When only glucose is available

Role of operator in prokaryotic

transcription (Lactose operon of E.coli)


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The tryptophan operon


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Tryptophan operon codes for 5 proteins that is required for

synthesis of tryptophan.

Just like lac operon, it is also subjected to both positive /

negative regulation.

Transcription attenuation in Prokaryotes vs Eukaryotes

Stringent response


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E.coli has 7 operons that synthesize rRNA needed for ribosomal assembly.

Regulation in response to amino acid starvation is known as Stringent response.

Uncharged tRNA bind with A site of ribosome ppGpp.

Role of stringent factor RelA.

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Regulatory ribosomal

protein


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Operon for ribosomal protein can be

inhibited by excess of its own product.

r-Protein does so by binding to Shine-

Dalgarno (SD) sequence located on

mRNA just upstream of the initiating

AUG codon.

How about the affinity of r-protein

for mRNA vs rRNA?

Regulation of Eukaryotic gene

expression


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Eukaryotic gene expression


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As with Prokaryotes, Eukaryote’s primary site of

regulation is Transcription.

There is no OPERON in Eukaryotes.

Eukaryotic gene regulation is regulated and

modified at several steps – even during protein

synthesis.

Combinatorial control of transcription


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Trans-acting molecules and Cis-acting regulatory elements

Transcription factor are multifactorial complex:

protein-protein interaction as well as protein-DNA complex

Histone acetyltransferase

Transcriptional regulation by intracellular

steroid hormone receptor


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GRE: Glucocorticoid-response element; GR: glucocorticoid receptor

Nuclear receptor superfamily: glucocorticoids, mineralocorticoids, sex

hormones, vitamin D, retinoic acid, thyroid hormone receptors.

Regulatory signal mediated by cell-

surface receptor


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Cell surface receptors: for insulin, epinephrine,

glucagon

Expression of protein through protein kinase A

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Regulation by co-and posttranscriptional

processing of mRNA


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Splice-site choice and mRNA editing

Over 60% of ~30,000 genes undergo differential splicing.

Use of alternative polyadenylation and transcription start

sites is also seen in many genes. 100,000 proteins

mRNA editing


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1. Happens with transcript of

apolipoprotein B gene (apoB)

2. apoB is essential protein component

of VLDL and chylomicrons

3. apoB mRNA is made in small intestine

and liver

4. Codon (CAA) for glutamine is

deaminated to UAA.

Regulation through modification to DNA


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Gene expression is affected by availability of DNA, amount of DNA and arrangement of DNA.

Access to DNA – euchromatin vs heterochromatin

Amount of DNA: gene amplification, drug methotrexate used against cancer is an inhibitor of enzyme dihydrofolate reductase required for TPP synthesis.

Arrangement of DNA

Mobile DNA elements: transposons (Tn)

Gene expression

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