The document discusses various mechanisms of gene expression regulation in prokaryotes and eukaryotes. In prokaryotes, gene expression often occurs in operons, clusters of genes that are regulated as a single unit. A key example is the lactose operon in E. coli, which codes for proteins involved in lactose metabolism. In eukaryotes, gene expression can be regulated at multiple stages including transcription, RNA processing, and translation. Regulatory mechanisms include transcription factors, histone modification, and alternative splicing of mRNA.

1. 11/23/2014
1
GENE EXPRESSION
Department of Biochemistry, Kathmandu Medical College
Sunday, November 23, 2014
1
Gene expression
Sunday, November 23, 2014
2
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
Sunday, November 23, 2014
3
Regulation controlled by regulatory sequence embedded in the non-coding region of DNA.
Cis-acting element vs Trans- acting element.
Polycistronic vs Monocistronic
Sunday, November 23, 2014
4
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
Sunday, November 23, 2014
5
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
Sunday, November 23, 2014
6
Consists of:
3 structural gene: LacZ, LacY, LacA
Promoter
Terminator
Regulator (Regulatory region)
Operator
Inducer molecules has a role in controlling gene expression. How?

2. 11/23/2014
2
The lactose operon
Sunday, November 23, 2014
7
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.
Sunday, November 23, 2014
8
Role of operator in prokaryotic transcription (Lactose operon of E.coli)
Sunday, November 23, 2014
9
When only glucose is available
Role of operator in prokaryotic transcription (Lactose operon of E.coli)
Sunday, November 23, 2014
10
The tryptophan operon
Sunday, November 23, 2014
11
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
Sunday, November 23, 2014
12
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.

3. 11/23/2014
3
Regulatory ribosomal protein
Sunday, November 23, 2014
13
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
Sunday, November 23, 2014
14
Eukaryotic gene expression
Sunday, November 23, 2014
15
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
Sunday, November 23, 2014
16
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
Sunday, November 23, 2014
17
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
Sunday, November 23, 2014
18
Cell surface receptors: for insulin, epinephrine, glucagon
Expression of protein through protein kinase A

4. 11/23/2014
4
Regulation by co-and posttranscriptional processing of mRNA
Sunday, November 23, 2014
19
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
Sunday, November 23, 2014
20
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
Sunday, November 23, 2014
21
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)