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Synthesis of proteins.Posttranslational modifications

of proteins.Regulation of gene expression.

Department of Biochemistry

Faculty of Medicine (E.T.)

2011

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Translation – synthesis of proteins

Which cells: all cells having nuclear DNA

Where in the cell: ribosoms (free or attached to ER) mitochondrias

Differences between prokaryotes and eukaryotes:

prokaryotes: transcription, processing of transcript and translation are not separated by space

eukaryotes: translation starts after the mRNA synthetized in nucleus is transported to cytoplasma

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Which molecules are necessary for synthesis of proteins ?

Amino acids

Many enzymes

Protein factors

ATP and GTP

Mg2+, K+ ions

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Genetic code

Proteins – 20 different AA RNA – 4 bases

Each amino acid is characterized by a triplet of bases in mRNA – codon

Codons consisting of three nucleotide can provide 64 codons → 61 of them code for 20 amino acids

3 codons are STOP codons (UAA,UAG, UGA)

Nierenberg (1961) – poly(U) sequence of mRNA produced polyphenylalanine by translation sequence UUU is the codon for phenylalanine

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First

base

Second base

Third

base

5´ U C A G 3´Phe Ser Tyr Cys UPhe Ser Tyr Cys C

U Leu Ser Stop Stop ALeu Ser Stop Trp G

Leu Pro His Arg ULeu Pro His Arg C

C Leu Pro Gln Arg ALeu Pro Gln Arg G

Ile Thr Asn Ser U A Ile Thr Asn Ser C Ile Thr Lys Arg A

Met Thr Lys Arg G

G Val Ala Asp Gly UVal Ala Asp Gly C

Val Ala Glu Gly AVal Ala Glu Gly G

Genetic codon

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• it is degenerate (more than one codon can code for an amino acid)

• it is unambiguous (each codonspecifies one and only one amino acid)

• it is almost universal ( the code is identical in prokaryotes and eukaryotes with some exceptions : e.g. human mitochondrial mRNA the triplet UGA codes for trp instead of stop-codonu, AUA codes for methionine instead leucinu)

• it can wobble (base pairing between the last nucleotide of the triplet and corresponding nucleotide is not strictly by Watson-Crick rule)

• it is nonoverlapping and „comaless“ (the codons are aligned without overlap and without empty spaces. Each base belongs to one codon and only one codon)

Prominent properties of genetic codon:

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Degeneracy of the genetic code

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Relation between mRNA and the protein product

•Sequence of bases in mRNA is sorted into codons

•The start codon (AUG) sets the reading frame

•The order of codons in the mRNA determines the linear sequence of amino acids in the protein – the reading is given by reading frame

……A U G C A C A G U G G A G U U……….

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The reading frames Only one of the three possible frames is the right one,it begins at the start codon AUG recognized by the Met-tRNAMet

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Efect of mutations

Mutations are structural alterations that result from damage of DNA molecules or unrepaired errors during replication

They can be transcribed into mRNA

By translation of altered base an abnomal sequence of amino acids can appear in the protein

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Types of mutation

1. point mutation exchange of single base

a) No detectable effect – because of degeneration of the code – silent mutation

e.g. CGACGG (both sequences code for Arg)

b) Missence effect – different amino acid is incorporated at the corresponding site in the protein molecule

e.g. GCACCA results in replacement of arg by prolinem

c) nonsense – they result in premature termination

e.g. CGA UGA, the codon for Arg is replaced by stop-codon

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Types of mutation (cont.)

2. insertion – one or more nucleotides are added to DNA

3. deletion – one or more nucleotides are removed from DNA

The damage of the protein depends on the number of deleted or inserted nucleotides

If three nucleotides (or more triplets) are inserted/deleted without a change of the reading frame, polypeptides with inserted/deleted amino acyl residues will be synthesized.

If one or two nucleotides are inserted/deleted, the result is a "frame-shift mutation„ that gives nonsense codons, distinct primary structure of proteins, etc.

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Example of point mutation

Point mutations in the genes for hemoglobin:

There is known about 800 of structural variants of human hemoglobin

Most of them results from point mutations and are not harmfull.

Some of them cause diseases.

Methemoglobinemia – e.g. replacement of single histidine by tyrosine in -chain protein is unattackable for the treatment of methemoglobin reductase, the methemoglobin in blood rises

Sickle cell anemia – missense mutation, GTG replaces GAG replacement of glutamate by valine in position 6 of -chain the chain become less soluble and precipitate when deoxygenated

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Amino cids cannot react directly with bases

„the adapter molecules are tRNAs“

• each molecule of tRNA contains anticodon

• anticodon is a triplet of bases that are complementar with the codon in mRNA

• each tRNA can bind specific AA on its 3´-terminal

The genetic code defines the relationship between the base sequence of the messenger RNA and the amino acid sequence of the polypeptide:

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General structure of tRNA molecules

The cloverleaf structure

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Formation of aminoacyl-tRNA

• aminoacid is firstly activated by a reaction of its carboxylic group with ATP to aminoacyl-AMP

• activated AA is transferred to 2´- or 3´- OH group of ribose on 3´terminal of tRNA

• reaction is catalyzed by specific enzymes (aminoacyl-tRNA synthetases)

• 20 different synthetases exist, one for each AA

• the cleavage of ATP in the first reaction provides energy

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N

N

N

N

N H 2

O

O H O H

OPO

O

O

Aminoacyl-AMP

(mixed anhydride)

+ ATPC OC

OH

NH2

R H

2Pi +

1. Activation of AA

CC

OH

NH2

R

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2. Transfer of activated AA to 3´-end of tRNA

Cyt

t-RNA

3´- end of t-RNAEster bond between -COOH of amino acid and 3´-OH of ribose

Amino acid

CO

CNH2

R

N

N

NH2

P

O

O-

N

N

CH2

O OH

OO

PO

O

O-CH2

O OH

OO

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(at least 20 distinct enzymes in cells) exhibit the very high degree of specifity for amino acids.

The enzyme molecule recognizes both a specific amino acid and a specific tRNA.

These enzymes discriminate accurately, the overall rate of occurence of errors in translating mRNA is less than 1 in 10 000.

This high specifity is oft called the 2nd genetic code.It depends on specific location of some bases in tRNA molecules,not on the sole anticodon.

Aminoacyl-tRNA synthetases

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Phases of translation

A. Inciation

B. Elongation

C. Termination

They occur in cytoplasma, in connection with ribosomes

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Ribosomes

Large ribonucleoprotein particles – composed of proteins and RNA

Free floating in the cytoplasma or attached to the membranes

Ribosomes are workbenches for protein synthesis

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Ribosomes

Ribosomes consist of a large and a small subunit

Inactive ribosomes exist as loose subunits that aggregate into comlete particles when they get ready for protein synthesis.

Large ribosomal subunit has three binding sites for molecules of tRNA – P, A, E

P-peptidyl-tRNA

A-aminoacyl-tRNA

E-free tRNA (exit)

E P A

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Prokaryotic x eukaryotic ribosomes

Property Bacterial Human

Sedimentation constants:

complete ribosome

small subunit

large subunit

RNA content

RNA-small subunit

RNA-large subunit

Placement in the cell

70S

30S

50S

65%

16S

5S 23S

Free floating in cytoplazma or attached to plasmatic membrane

80S

40S

60S

50%

18S

5S 5,8S 28S

Free floating in cytoplazma or attached to ER membrane

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Initiation

Ribosome has to associate with mRNA and the initiator tRNA

Formation of initiation complex.

Soluble cytoplasmic factors helps in initiation – initiation factors

Also GTP, ATP are involved

Differences between eukaryonic and prokaryonic cells

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Initiation u eukaryotes

5´-P-P-P-5´-5´ G

Small ribosomal subunit

Large ribosomal subunit

Guanine cap

mRNA

E P A

GIniciation factors

It involves formation of a complex composed of methionyl t-RNAmet, mRNA and a ribosome.

CH2N

R

H

C O

O

antikodon

Aminocyl-tRNA

tRNA

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40S

eIF3eIF1A

Binding of factors eIF3 and eIF1A to a small subunit

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G

eIF2

GTP

AMP

Met

CH2N

R

H

C O

O

3´

Binding of activated Met to tRNAMet Binding of GTP to eIF2

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Met

Factor eIF2 recognizes the initiation aminoacyl-tRNA

CH2N

R

H

C O

O 3´

G

GTP

eIF2

UAC

Binding of complex GTP-eIF2 to t-RNAMet

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m-RNA

A C A U G U U G C C…...5´-P-P-P-5´-5´ G

CBP (cap binding protein) binds to the cap on 5´end of mRNA

CPB (eIF-4F) is composed of a number of other initiation factors (eIFs)

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CH2N

R

H

C O

O

G

GTP

eIF2 UAC

Met

Complex Met-tRNAMet, eIFs and GTP binds to the snaller ribosomal subunit

Preiniciation complex

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CH2N

R

H

C O

O

G

GTP

eIF2 UAC

A C C G U A A C A U G U U G C C G5´-P-P-P-5´-

m-RNA

Binding of m-RNA to preiniciation complex

Reaction requires ATP for unwinding a hairpin loop in the mRNA (helicase activity of an eIF subunit)

The complex scans mRNA from 5´end until it locates the AUG start codon

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CH2N

R

H

C O

O

UAC

A C C G U A A C A U G U U G C C G5´-P-P-P-5´-

APE

• GTP is hydrolyzed•eIF separates

Pi + GDP

eIF

•The larger ribosomal unit is attached

•Met-tRNA bimds at the site P of larger ribosomal subunit

Met

Iniciation complex 80S

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Differences in between prokaryotes and eukaryotes

eukaryotes prokaryotes

Binding of mRNA to smaller ribosomal subunit

Cap on the 5´end of mRNA binds IFs and 40S subunit containing t-RNAmet.

mRNA is scanned until AUG

No cap, Shine-Dalgarno sequence in mRNA about 10 nucleotides upstream of the AUG start codon is attached to a complementary sequence in 16S RNA

First AA methionine formylmethionine

Iniciation factors 12 and more 3

ribosomes 80s (40s a 60s) 70s (30s a 50s)

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eIF2 factor in eukaryotes

eIF2 has essential significance for initiation of translation – control point in proteosynthesis

Its phosphorylated form is inactive.

Conditions such starvation, heat shock, viral infection, glucose starvation result in phosphorylation of eIF2 by specific kinases (generally conditions when the energy expanditure required for synthesis of proteins would be deleterious)

Example of regulation:

Synthesis of globin in v reticulocytes

• In the absence of heme, the eIF2 is phosphorylated, the rate of initiation of globin synthesis decreases

• heme acts by inhibiting the phosphorylation of the eIF2 eIF2 is active in the presence of heme and globine synthesis is initiated

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Elongation of peptide chain

• formation of the second aminoacyl-tRNA

• binding of an aminoacyl-tRNA to the A site on the ribosome

• formation of peptide bond

• translocation of peptidyl-tRNA to the site P

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CH2N

R

H

C O

O

UAC

APE

Antikodon is AAC

Amino acids is leucin

A C C G U A A C A U G U U G C C G5´-P-P-P-5´-

Which amino acid will be added? The next codon is UUG

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GGTP

EF1

CH2N

R

H

C O

O

G

GTP

EF1 AAC

Leu

1. Activation of leucine by reaction with ATP leucinyladenylate

2. formation of Leu-tRNA

3. + binding of GTP a EF1

Formation of Leu-tRNA

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UAC

APE

CH2N

R

HC OO

A C C G U A A C A U G U U G C C G5´-P-P-P-5´-

CH2N

R

HC OO

AAC

Met Leu

Leu-tRNA binds to the site A

APE

GTP is hydrolyzed to GDP + Pi, complex GDP-EF1 is released

GDP-EF1 Pi

Proces elongace je u prokaryotů a eukaryontů velmi podobný (odlišné kofaktory elongace)

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UAC

APE

A C C G U A A C A U G U U G C C G5´-P-P-P-5´-

AAC

MetLeu

APE

CHN R

HC

OO

CH 2N

R

HC

O

Binding site on tRNAMet is empty

Peptidyltransferasa is rRNA. It is a component of 28S RNA subunit 60S – ribozyme activity

Formation of peptide bond (transpeptidation)

Synthesis of proteins starts with N-terminal

Peptidyltransferase catalyzes the release of methionine from tRNA and its transfer to leucine. A peptide bond between carboxyl group of methionine and amino group of leucine is formed

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UAC

APE

A C C G U A A C A U G U U G C C G5´-P-P-P-5´-

AAC

MetLeu

APE

CHN R

HC

OO

CH 2N

R

HC

O

Movement of met-tRNA to the site E

1. EF2 and GTP bind to ribosome

EF2 + GTP

2. tRNAMet moves to the site E, site P become free

G

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APE

A C C G U A A C A U G U U G C C G5´-P-P-P-5´-

AAC

MetLeu

APE

CHN R

HC

OO

CH 2N

R

HC

O

Result of animation of page 42

UAC

G

Movement of met-tRNA to the site E

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APE

A C C G U A A C A U G U U G C C G5´-P-P-P-5´-

AAC

Met

Leu

APE

CHN R

HC

OO

CH 2N

R

HC

O

Translocation and release of Met-tRNA

GDP-EF2 + Pi

Movement of ribosome with respect to mRNA and its base-paired tRNAs peptidyl-tRNA moves into the P site

Site A is emptyGTP is hydrolyzed, EF2 will release

Direction of movement

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Which further steps will follow?

Next step of elongation

Further tRNA charged with amino acid (proline) binds to the site A

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Termination

• elongation steps are repeated until a termination (stop) codon moves into the A site of the ribosome

• no tRNA that can pair with stop codon is present in cytoplasma

• releasing factors bind to ribosome instead

• peptidyltransferase hydrolyzes the bond between peptide chain and tRNA

• newly synthesized peptide is released from ribosome

• ribosome dissociates into individual subunits, mRNA releases

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Energy consumption

Aaminoacyl-tRNA formation ATP AMP + 2 Pi 2

Binding of aminacyl-tRNA to the site A GTP GDP + Pi 1

Translocation of peptidyl-tRNA to the site P GTP GDP + Pi 1

Equivalent of ATP

4 ATP

4 ATP are required for synthesis of one peptide bond.

Further energy is required for initiation and synthesis of nukleotides.

Proteosynthesis rate

Prokaryotes ~ 100 peptide bond s

Eukaryotes ~100 peptide bonds/min

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Stimulation of proteosynthesis by insulin

CBP (cap binding protein complex) has subunit eIF4E

This subunit is blocked by protein 4E-BP

Insulin triggers phosphorylation of 4E-BP

Phosphorylated 4E-BP looses its afinity to eIF4E

eIF4E become free for participation in protein synthesis

Insulin is anabolic hormone, it stimulates proteosynthesis.

It affects the synthesis through the interaction with CBP.

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Polysomes

NH2

NH2

NH2

NH2

As one ribosome moves along the mRNA produsing a polypeptide chain, a second ribosome can bind to the vacant 5´-end of mRNA. Many ribosomes can simultaneously translate a single mRNA, forming a complex known as polysome. A single ribosome covers approximately 80 nucleotides of mRNA. Therefore, ribosomes are positioned on mRNA at intervals of approximately 100 nucleotides.

Simultaneous translation of mRNA on more ribosomes

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Synthesis of proteins in mitochondria

•Mitochondria contain 2-10 copies of circular double stranded DNA

•The size varies depending on the species

•Animal mitochondrial DNA - Mr~ 107

•Codes for rRNA, set of tRNAs and mRNA for several proteins

•Proteins synthesized in mitochondrial are a negligible proportion of total proteins of inner mitochondrial membrane but are essential for process of oxidative phosphorylatiob ( a part of complexes I,III,IV and ATP-synthase)

•Synthesis of of proteins in mitochondria has many common features with synthesis in prokaryotes (e.g. initiation by formylmethionine, sensitivity to antibiotics)

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Antibiotics Effect

Streptomycine Binds to 30S ribosomal subunit, inhibits formation of initiation complex.Triggers errors in reading frame of mRNA.

Tetracycline Binds to 30S ribosomal subunit and inhibits binding of aminoacyl-tRNA to site A

Chloramfenikol Binds to 50S ribosomal subunit and inhibits peptidyltransferase

Erytromycine Binds to 50S ribosomal subunit and inhibits translocation

Puromycine Binds to A-site on ribosome and trigers premature termination

Effects of antibiotics on prokaryotic proteosynthesisthe differences in proteosynthetic procedure between eukaryotes and bacterias are exploited for clinical purposes

some antibiotics inhibits specifically proteins of bacterial ribosomes

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•Nascent polypeptide chain is transported across ribosomes

• Outside of the ribosome the N-terminus of the protein begins to fold (while the C-terminal portion of the protein is still being synthesized by the ribosome)

• Folding is the process by which a protein achieves its thermodynamically-stable three-dimensional shape to perform its biological function

•Specialized proteins called chaperones assist in the folding.

Incorrect protein folding is associated with some diseases - Alzeimer disease, BSE, cystic fibrosis etc..

Folding of proteins

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Posttranslational processing of proteins

• Methionine removal

•ADP-ribosylation

•Glycosylation

•Fatty acylation

•Phosphorylation

•Acetylation

After protein emerge the ribosome, they may undergo posttranslational modification

•Carboxylation

•Methylation

•Prenylation

• Hydroxylation

•Sulfatation ad.

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Targeting of proteins to subcelular and extracellular locations

Synthesis of proteins on polysomes in cytosol

Proteins remain in the cytosol or enters the organeles (nucleus, mitochondria). They contain amino acid sequence (targeting sequence) that facilitate their transport into a certain organelle

Synthesis of proteins on ribosom bound to RER

Transport to lysosomes, ER, Golgi complex, cellular membranes or secretion

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Transport of proteins syntesized on RER

RER membrane

Signal peptide (ussualy 15-30 hydrophobic amino acids on N-terminl end)

Signal recognition particle (SRP)

SRP-receptor Signal

peptidaseCleaved signal peptide

1.

2.

3

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6

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1. translation begins in the cytosol

2. As the signal protein emerges from the ribosome, a signal recognition particle (SRP) binds to it and to the ribosome snd inhibits further synthesis of the protein

3. the SRP binds to the SRP receptor in the RER membrane, docking the ribosome on the RER

4. The SRP is released and protein synthesis resumes

5. As the signal peptide moves through a pore into the RER, a signal peptidase removes the signal pepide

6. Synthesis of the nascent protein continues and the completed protein is released into the lumen RER

Transport of proteins syntesized on RER

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LyzosomesSecretory vesicles

Cis Golgi

Trans Golgi

http://vcell.ndsu.nodak.edu/animations/proteintrafficking/first.htm

Transport of proteins syntesized on RER

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• Proteins syntesized ribosomes attached to RER travel in vesicles to the cis face of the Goolgi complex

• here is the sorting center – structural features of proteins determine their direction

• some remain in Golgi complex, some return to RER

• others bud from trans face of the Golgi complex in vesicles

• these vesicles become lysosomes or secretory vesicles depending on their contents

• secretory proteins are released from the cells when secretory vesicles fuse with the membranes

• proteins with hydrophobic regions embedded in the membrane of secretory vesicles become cell membrane proteins

Transport of proteins syntesized on RER

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Principles of intracellular sorting

Example 1:

Proteins determined for lysosomes are marked by N-bonded oligosacharides terminated by mannose-6-P

Prot-oligosaccharide-mannose-6-P „address“ is recognised

by specific membrane receptors in Golgi complex that embedds the protein into the clathrine coated vesicles

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Example 2:

Proteins destinated for ER have sequence Lys-Asp-Glu-Leu on their carboxy terminal

lys-asp-glu-leu Proteins are transported back from Golgi complex to ER

Principles of intracellular sorting

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Glycosylation of proteins

Glycoproteins

N-linked carbohydrate chain

Involving the amide nitrogen of asparagine

They differ in composition of saccharides and and the way of synthesis

O-linked carbohydrate chain

Involving hydroxyl side chain of serine or threonine

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Synthesis of N-glycoproteins

The oligosaccharide chain is first assembled on the dolichodiphosphate backbone

Dolichol is isoprene (see the synthesis of cholesterol)

H-[CH2-C=CH-CH2]n-CH2-CH-CH2-CH2OH

CH3 CH3

n=18-20

Dolicholdiphosphate is bonded to membrane of ER.

Activated monosaccharides are gradually attached to the terminal phosphate.

The oligosaccharide chain is then transferred en bloc to suitable Asn residues of apoglycoproteins during their synthesis on membrane-bound polyribosomes

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Dolichol-P

Dolichol-P-P←GlcNAc←GlcNAc

Dolichol-P-P←(GlcNAc)2←(Man)5

Dolichol-P-P←(GlcNAc)2 (Man)9

Dolichol-P-P←(GlcNAc)2 (Man)9 (Glc)3

2 UDP-GlcNAc

5 GDP-Man

4 Dolichol-P-Man

3 Dolichol-P-Glc

Synthesis of oligosaccharide precursor

Transfer to protein

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Trimming and final processing of N-linked glycoproteins:

(Glc)3 (Man)9 (GlcNAc)2-protein

– 3 Glc

(Man)9 (GlcNAc)2-protein

(Man)5 (GlcNAc)2-protein

(Man)3 (GlcNAc)2-proteinpentasaccharide core region

High-mannoseN-linked glycoproteins

up tp 4 Manremoved

– 2 Man

glycosylation

Hybride types

Complex typeN-linked glycoproteins

glycosylation

Late phase processing

Trimming

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Complex type(triantennary chain)

Hybride type(one antenna)

High-mannose type(before trimming to core region)

Examples of plasma-type (N-linked) oligosaccharidesThe boxed area encloses the pentasaccharide core region common to all N-linked glycoproteins.

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Synthesis of O-glycoproteins

………Ser…………….………Ser…………….

OH UDP-monosach. UDP O-monosacharide

is posttranslational process that takes place exclusively in the Golgi complex and which is direct – glycosyls from nucleotide sugars (NuDP-glycoses) are transferred to side chains of Ser or Thr residues and elongated by other nucleotide sugars

(Thr) (Thr)

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Regulation of gene expression

Gene expression – formation of proteins or RNA products

Generally only small fraction of genes in a cell are expressed at any time

Gene expression is regulated differently in prokaryotes and eukaryotes

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Circular DNA

mRNA

•only one DNA in the cell

•DNA is not complexed with histones

•nucleus is not separated from cytoplasma

•transkripts of genes do not include introns

•translation and transcription occur simultaneously

The main features of gene expression in prokaryotes

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Regulation of gene expression in prokaryotes

Regulation is less complex tham the multicellular eukaruotes

Gene expression is regulated mainly by controlling the initiation of gene transcription.

The most extensive studied is bacterium E.coli. Its genom includes 4x106 base pairs E.coli should be able of making several thousands of proteins

Under normal conditions they synthesize only about 600-800 different proteins.

Many genes are inactive and only those genes are expressed that generate the proteins required for growth in that particular enviroment

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Housekeeping genes and inducible genes

Expression of some genes is constitutive – they are expressed at a reasonably constant rate and are not the subject of regulation - housekeeping genes

Some genes are inducible - their expression requires an inducer or activator

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Operons theory

Structural gens of bacterias are grouped into units called operons

promotor

1 32

Structural genes

operon

AUG AUGUAA AUGUGA UAG5´

Prot 1 Prot 1 Prot 1

DNA

3´mRNA

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Operon

• it includes structural genes for proteins that are metabolically related

•genes in operon are ussually coordinately expressed (they are either all „turned on“, or all „turned off“)

•The product of transcription is single polycistronic mRNA

•transcription is regulated by single promotor which is located in the operon at the 5´-end, upstream from the structural genes

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Regulation of RNA polymerase binding by repressors – negative control

Regulatory gene

DNA

mRNA

repressor

A B C

operatorstructural genes

repressor je encoded by regulatory gene

Its product, the repressor protein, difuses to the promoter and binds in the region of the operon called operator

Operator is located within the promoter or near of its 3´-end

Repressor blocks the binding of RNA-polymerase to the promotor

Synhesis of mRNA does not occur

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Repressor is controlled by two mechanisms

Induction Corepression

Inductor is a small molecul that binds to repressor, changes its conformation and triggers its release from the operator

transcription can start

Inductors: small molecules of nutrients or their metabolites

repressor is not active until corepressor is bonded to it. The complex repressor-corepressor binds to operator preventing binding of RNA polymerase transcription stops

Corepressors: small molecules of nutrients or their metabolites

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Example of induction

Induction of lac operon in E.coli by lactose

Enzymes for metabolizing glucose by glycolysis are produced constitutively

If the milk sugar lactose is available, the cell adapt and begin to produce three additional enzymes required for lactose metabolism

These enzymes are encoded by lac operon

A metabolite of lactose (allolactose -isomer of lactose that is formed spontaneously) serves as an inducer, binds to the repressor and inactivates it.

RNA polymerase can bind to promotor and transcribe the structural genes of the lac operon (-galactosidase, permease and transacetylase)

Glucose can prevent activation of the lac operon (see fig.80)

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Example of corepression

Corepression of trp operon (synthesis of tryptophane at E.coli)

genes for enzymes of tryptophane synthesis (5 enzymes) are located in trp operon

Tryptophan ise corepresssor, it binds toinactive repressor, binds its conformation.

The complex tryptophan-repressor inhibits transcription of operon.

http://www.biology.ualberta.ca/facilities/multimedia/uploads/genetics/trpoperon2.swf

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Stimulation of RNA polymerase binding - positive control

Regulatory proteins binds to promoter and stimulate the binding of RNA-polymerase

Regulatory protein is activated on the base of presence/or absence of small molecule of nutrient of its metabolite in the cell

Catabolite repression

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Example of positive control

Transcription of lac operon at E.coli

transcription of lac operon is affected by allolactose only when the glucose is absent

Decrease of glucose level results in increase of cAMP (it is not known why)

cAMP binds to its receptor in the cell (cAMP-receptor protein → CRP)

Complex cAMP-CRP binds to the regulatory site of lac operon, stimulates the binding of RNA polymerase to promotor and transcription of genes for

metabolism of lactose

→ cells metabolize lactose, only when it does not have adequate supply of glucose

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Atenuation of transcription

Some operons are regulated by process that interrups (attenuates) transcription after it has been initiated

The cause is the change of secondary structure of mRNA

Attenuator („retarder“ ) is a sequence included in operon

Processes of translation occurs at the same time with transcription

The rate of transcription affects the formation of hair-pin loops on mRNA

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Atenuation of transcription – trp operon E.coli

1 2 3 4

1

2 3

4

1 2

Low conc. of Trp

High conc. of Trp STOP

Ribosom is attenuated

Quickly moving ribosome

Sequence 1 contains codones for Trp

5´mRNA

80

• mRNA is transcripted from the trp operon, ribosomes bind to it and rapidly begin to translate the transcript.

• RNA polymerase is followed by moving ribosome

• near the 5´end of the transcript there are number of codons for trp.

• initially, high levels of trp in the cell result in high levels of trp-tRNAtrp and rapid translocation of the transcript

• rapid translocation generates a hairpin loop in the mRNAthat serves as a termination signal for RNA polymerase and transcription terminates.

• when trp levels are low, levels of trp-tRNAtrp are low and ribosome stall at codons for trp. A different hairpin loop forms in mRNA that does not terminate transcription

•RNA polymerase can complete the transcription

•The level of transcription is regulated by the amount of trp in the cell

Atenuation

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Regulation of protein synthesis in eukaryotes

The main features of gene expression in eukaryotes (differences from prokaryote):

• DNA is organized in nucleosomes of chromatin

• gene must be in an active structure to be exxpressed in a cell

• operons are not present

• genes encoding metabolically related proteins are located on different chromosomes

• each gen has its promotor

• transcription and translation are separated

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Regulation of eukaryotic cells gene expression occurs at multiple level

•A) DNA and the chromosome including chromosome remodeling and gene rearrangement

•B) transcription, primarily through transcription factors affecting binding of RNA polymerase

•C) processing of transcripts

•D) initiation of translation and stability of mRNA

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Regulation of availability of genes for transcription

Chromatin in the nucleus:

Condensed (heterochromatin ) – genes are inactive

Euchromatin- genes produce mRNA

In cells of differentiated tissues only the genes that have some role in the cell are active

Long-term changes in the activity of genes occur during development as chromatin goes from a diffuse to a condensed state and vice versa.

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A) Examples of regulation on the chromosom structure level

Chromatin remodeling

DNA methylation

gene rearangement

gene amplification

gene deletion

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Chromatin remodeling

- change of chromatin state that results in activation of transcription

displacement of of nucleosome from chromatin so that the transcription can start

Remodeling mechanisms:

• an ATP driven unwinding of certain section of DNA from nuclosome core

• covalent modification of the histone tails through acetylation or deacetylation (acetylation of -amino group in side chain of lysin on N-terminals of histones H2A,H2B,H3 a H4).This reaction removes a positive charge from the --amino group of the lysine, thereby reducing the electrostatic interactions between the histones and and the negatively charged DNA this results in an easier unwinding

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DNA methylation

Methylation of cytosine residues in DNA 5-methylcytosine

Methylations are located in GC-rich sequences (GC-islands) that are often near or in the promoter region of gene (postsyntetic modifiation of DNA – enzym methylase)

Genes that are methylated are less readily transcribed

Example: globin genes are more extensively methylated in nonerythroid cells than in cells in which these genes are expressed (erytroblasts and retikulocytes)

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Gene rearrangement

Segments of DNA can move from one location to another in the genome, associating with each other in various ways

Example : rearrangement of genes in cells that produce antibodies (imunoglobulins)

(see Imunology)

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Gene amplification

Certain region of a chromosome undergo repeated cycles of DNA replication

The newly synthesized DNA is excised and forms small, unstable chromosomes called double minutes.

They intgrate into other chromosomes thereby amplifying the gene in the process.

It is not usual physiological means of regulating of gene expression in normal cells, it occurs in response to certain stimuli

Normally gene amplification occurs through errors during DNA replication and cell division – then cells containing amplified genes may have a grow advantage, if the enviromental conditions are appropriate.

Example: patients treated by methotrexate (nhibitor of dihydrofolate reductase) can develop drug resistance.

The cause is that some rapidly deviding cancer cells amplify the gene for dihydrofolate reductase, producing hundreds of copies in the genome. These cells generate large amount of dihydrofolate reductase and normal doses of methotrexate are not longer adequate.

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B) Regulation at the level of transcription

Basal regulation of transcription (common for all genes)

Regulation by the components of „basal transcription complex“ (RNA polymerase binding the TATA box, TATA binding proteins and further basal transcription factors binding on promoter or RNA-polymerase)

Genes regulated only in this way:

constitutively expressed genes

Specific regulation of gene expression:

Gene specific transcription factors bind to the specific regulatory sequences.

See the lecture 13

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Regulation of transcription factors

Down(up)-regulation

Modulation by binding of stimulatory and inhibitory ligands

Mutual cooperation of transcription factors

Phosphorylation/ dephosphorylation of transcription factors regulated growth actors, cytokines, peptide hormones atd.

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C) Postranscriptional regulation

Alternative splicing

Alternative splicing and variation of the site of polyadenylation cause that one gene can produce various proteins (see the lecture 13)

RNA editing

In some instances, RNA is „edited“ after transcription.

Primary transcript and the sequence of gene are the same, but bases are altered or nucleotides are added or deleted after the transcript is synthesized.

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RNA editing

synthesis of apoB in hepatocytes and enterocytes

CAA Gen apoB

transcriptCAA

hepatocyte enterocyte

CAAmRNA UAA

Stop!translation

Liver apoB -4563 AA Intestinal apoB – 2152 AK

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Synthesis apoB in hepatocytes and enterocytes

(apo B is a component of chylomicrons and VLDL)

Gen apoB produces protein containing 4563 amino acids in liver

The same gene produces apoB containing only 2152 amino acids in enterocytes

Conversion of C(cytosin) to U (uracil) by deamination of mRNA transcript generates the stop-codon in intestinal mRNA. Thus the protein formed in the enterocyte has only 48% of lenght in comparision with apoB in liver

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D) Regulation at the level of translation

Regulation ussualy involves the initiation of protein synthesis by eIFs (eukaryotic initiation factors)

See Fig. 34,46