Regulation of Gene Expression in Eukaryotes : 1 Regulation of Gene Expression in Eukaryotes How does a eukaryotic organism regulate the expression of gene leading to the production of correct protein?
Expression of Different Genes : 2 Expression of Different Genes House keeping genes
Genes required during cellular differentiation
Genes which get triggered as a response to some external factors
Genes which get triggered during apoptosis
Mechanism of Gene Regulation in Prokaryotes and Eukaryotes : 3 Mechanism of Gene Regulation in Prokaryotes and Eukaryotes In prokaryotes the primary control point is the process of transcription initiation
In eukaryotes expression of gene into proteins can be controlled at various locations.
Check Points for Gene Expression in Eukaryotes : 4 Check Points for Gene Expression in Eukaryotes Synthesis of proteins is controlled right from the chromatin stage.
Expression of gene is controlled at many steps during the process of transcription and translation.
Description of the control points is dealt in detail in the subsequent slides.
1.Chromatin Structure : 5 1.Chromatin Structure Two forms of chromatin
Euchromatin – A lesser coiled transcriptionally active region which can be easily accessed by the RNA polymerases.
Heterochromatin – A highly condensed transcriptionally inactive region. The genes in this region cannot be accessed by the RNA polymerases for active transcription.
1.Chromatin Structure : 6 1.Chromatin Structure Mechanisms which affect the chromatin structure and hence the expression of gene are:
Histone modifications – These modifications make a region of gene either transcriptionally active or inactive.
Acetylation
↑Acetylation ----↓ Condensation of DNA ----- ↑ Transcription of genes in that region
1.Chromatin Structure : 7 1.Chromatin Structure Methylation
Methylation of histone H4 on R4 (arginine residue at the 4th position) →→ opens the chromatin structure →→ leading to transcriptional activation
Methylation of histone H3 on K4 and K79 (lysines residues at the 4th and 79th position) →→ opens the chromatin structure →→ leading to transcriptional activation
Methylation of histone H3 on K9 and K27 (lysines residues at the 9th and 27th position) →→ condenses the chromatin structure →→ leading to transcriptional inactivation
1.Chromatin Structure : 8 1.Chromatin Structure Ubiquitination
Ubiquitination of H2A – Transcriptional inactivation
Ubiquitination of H2B - Transcriptional activation
2) Methylation of DNA
Target sites of methylation are - The cytidine residues which exist as a dinucleotide, CG (written as CpG)
↑methylated cytidine -- ↓Transcriptional activity
2.Regulation of Transcription : 9 2.Regulation of Transcription The differences in the mechanisms by which the transcription of gene is controlled in prokaryotes and eukaryotes are listed below:
2.Regulation of Transcription : 10 2.Regulation of Transcription
2.Regulation of Transcription : 11 2.Regulation of Transcription Promoters
Prokaryotes - There are two promoter elements or DNA sequences which are 35 and 10 base pairs in length and seated upstream to the transcriptional initiation sites.
The consensus sequence present at
-35 position is TTGACA
-10 position is TATAAT. This is also termed as Pribnow-box.
Eukaryotes – There are two types of promoters which are:
Basal promoters
Upstream promoters
2.Regulation of Transcription : 12 2.Regulation of Transcription Basal promoter or core promoter -These promoters reside within 40bp upstream of the start site. These promoters are seen in all protein coding genes. Examples are CCAAT-boxes and TATA-boxes
TATA box
The consensus sequence for TATA box is TATAT/AAT/A
It resides 20 to 30 bases upstream of the transcriptional start site
This is similar in sequence to the prokaryotic Pribnow-box
Proteins like TFIIA, B, C interact with this TATA box
2.Regulation of Transcription : 13 2.Regulation of Transcription 2. CCAAT-box
The consensus sequence for this is GGT/CCAATCT
It resides 50 to 130 bases upstream of the transcriptional start site
Protein named as C/EBP (CCAAT-box/Enhancer Binding Protein) binds this box
2.Regulation of Transcription : 14 2.Regulation of Transcription Upstream promoters - These promoters may lie up to 200bp upstream of the transcriptional initiation site. The structure of this promoter and the associated binding factors keeps varying from gene to gene.
Enhancers
Enhancers can be located upstream, downstream or within the gene that is transcribed
The binding of these enhancers with enhancer binding proteins (transcription factors) increases the rate of transcription of that gene to a greater extent.
Promoters are capable of initiating lower levels of transcription.
Enhancers are responsible for the cell or tissue specific transcription.
Each enhancer has its own transcription factor that it binds to.
2.Regulation of Transcription : 15 2.Regulation of Transcription Action of an enhancer – An enhancer binding protein has two binding sites
Binds DNA
Binds the transcription factors that are bound to the promoter
Slide 16 : 16 How is the gene transcription controlled at this point?
The unique combination of the promoter sites, transcription factors and enhancers chosen ultimately decides which gene gets switched on and which one gets switched off.
3.Regulation of RNA Processing : 17 3.Regulation of RNA Processing RNA processing involves
Addition of 5' cap
Addition of a 3' poly (A) tail
Removal of introns
The RNAs which get translated to proteins are transported out from the nucleus to cytoplasm.
Depending on the final combination of exons after splicing different kinds of proteins are obtained which can perform different functions in the cell.
Exon Shuffling : 18 Exon Shuffling The functions of two proteins synthesized from the same mRNA are different in different cells as different combination of exons exist in different cells.
4.Regulation of RNA Transport : 19 4.Regulation of RNA Transport Only some RNAs function within the nucleus whereas all other RNAs which are meant for protein synthesis have to be transported from the nucleus to the cytoplasm via nuclear pores.
5.Regulation of RNA Longevity : 20 5.Regulation of RNA Longevity mRNAs from different genes have different life spans.
The information of the life span of mRNA is found in the 3' UTR.
The sequence AUUUA within 3' UTR acts as a signal for early degradation.
More the number of times the sequence is repeated Shorter the lifespan of mRNA
6.Regulation of Translation : 21 6.Regulation of Translation Translational initiation
The expression of a gene product also depends on the ability of the ribosome to recognize the correct AUG codon out of the multiple methionine codons present in the mRNA.
Control of translational process
In many animals large amounts of mRNAs are produced by the eggs but all of them do not get translated until the egg is fertilized.
7.Post Translational Control Points : 22 7.Post Translational Control Points Post translational modifications
Functional state of protein depends on modifications like glycosylation, acetylation, fatty acylation, disulfide bond formations.
Chaperons
Protein transport
Transportation to the site of action
Protein stability
The lifespan of a protein depends on the specific amino acid sequence present within them
Summary of the Class : 23 Summary of the Class The expression of genes is controlled at various levels in eukaryotes.
At the chromatin stage the level of condensation determines whether the genes will remain transcriptionally active or not.
The unique combination of the promoter sites, transcription factors and enhancers regulates the transcriptional rate of a gene.
After transcription the gene expression is controlled by RNA processing.
The expression of gene is also controlled at the level of translation and after translation.
Slide 24 : 24 Thank you