PowerPoint Presentation Materials to accompany

PowerPoint Presentation Materials to accompany
Genetics: Analysis and Principles Robert J. Brooker CHAPTER 15 GENE REGULATION IN EUKARYOTES Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

INTRODUCTION Eukaryotic organisms have many benefits from regulating their genes For example They can respond to changes in nutrient availability They can respond to environmental stresses In plants and animals, multicellularity and a more complex cell structure, also demand a much greater level of gene expression 15-2 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

INTRODUCTION Gene regulation is necessary to ensure
1. Expression of genes in an accurate pattern during the various developmental stages of the life cycle Some genes are only expressed during embryonic stages, whereas others are only expressed in the adult 2. Differences among distinct cell types Nerve and muscle cells look so different because of gene regulation rather than differences in DNA content Figure 15.1 describes the levels of gene expression that are regulated in eukaryotes 15-3 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

15.1 REGULATORY TRANSCRIPTION FACTORS
Transcription factors are proteins that influence the ability of RNA polymerase to transcribe a given gene There are two main types General transcription factors Required for the binding of the RNA pol to the core promoter and its progression to the elongation stage Are necessary for basal transcription Regulatory transcription factors Serve to regulate the rate of transcription of nearby genes They influence the ability of RNA pol to begin transcription of a particular gene 15-5 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Regulatory transcription factors recognize cis regulatory elements located near the core promoter
These sequences are known as response elements, control elements or regulatory elements The binding of these proteins to these elements, affects the transcription of an associated gene A regulatory protein that increases the rate of transcription is termed an activator The sequence it binds is called an enhancer A regulatory protein that decreases the rate of transcription is termed a repressor The sequence it binds is called a silencer Refer to Figure 15.2 15-6 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Structural Features of Regulatory Transcription Factors
Transcription factor proteins contain regions, called domains, that have specific functions One domain could be for DNA-binding Another could provide a binding site for effector molecules A motif is a domain or portion of it that has a very similar structure in many different proteins Figure 15.3 depicts several different domain structures found in transcription factor proteins 15-8 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

The recognition helix recognizes and makes contact with a base sequence along the major groove of DNA Hydrogen bonding between an a-helix and nucleotide bases is one way a transcription factor can bind to DNA Figure 15.3 15-9 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Composed of one a-helix and two b-sheets held together by a zinc (Zn++) metal ion
Alternating leucine residues in both proteins interact (“zip up”), resulting in protein dimerization Two a-helices intertwined due to leucine motifs Homodimers are formed by two identical transcription factors; Heterodimers are formed by two different transcription factors Note: Helix-loop-helix motifs can also mediate protein dimerization Figure 15.3 15-10 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Enhancers and Silencers
The binding of a transcription factor to an enhancer increases the rate of transcription This up-regulation can be 10- to 1,000-fold The binding of a transcription factor to a silencer decreases the rate of transcription This is called down-regulation 15-11 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Enhancers and Silencers
Many response elements are orientation independent or bidirectional They can function in the forward or reverse orientation Most response elements are located within a few hundred nucleotides upstream of the promoter However, some are found at various other sites Several thousand nucleotides away Downstream from the promoter Even within introns! 15-12 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

TFIID and Mediator Most regulatory transcription factors do not bind directly to RNA polymerase Two common protein complexes that communicate the effects of regulatory transcription factors are 1. TFIID 2. Mediator Refer to Figure 15.4 15-13 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

A general transcription factor that binds to the TATA box
Recruits RNA polymerase to the core promoter Transcriptional activator recruits TFIID to the core promoter and/or activates its function Thus, transcription will be activated Transcriptional repressor inhibits TFIID binding to the core promoter or inhibits its function Thus, transcription will be repressed Figure 15.4 15-14 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

STOP Transcriptional activator stimulates the function of mediator This enables RNA pol to form a preinitiation complex It then proceeds to the elongation phase of transcription Transcriptional repressor inhibits the function of mediator Transcription is repressed Figure 15.4 15-14 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Regulation of Regulatory Transcription Factors
There are three common ways that the function of regulatory transcription factors can be affected 1. Binding of an effector molecule 2. Protein-protein interactions 3. Covalent modification Refer to Figure 15.5 15-16 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

15-17 Figure 15.5 The transcription factor can now bind to DNA
Formation of homodimers and heterodimers Figure 15.5 15-17 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Steroid Hormones and Regulatory Transcription Factors
Regulatory transcription factors that respond to steroid hormones are termed steroid receptors The hormone actually binds to the factor The ultimate effect of a steroid hormone is to affect gene transcription Steroid hormones are produced by endocrine glands Secreted into the bloodstream Then taken up by cells 15-18 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Steroid Hormones and Regulatory Transcription Factors
Cells respond to steroid hormones in different ways Glucocorticoids These influence nutrient metabolism in most cells They promote glucose utilization, fat mobilization and protein breakdown Gonadocorticoids These include estrogen and testosterone They influence the growth and function of the gonads Figure 15.6 shows the stepwise action of a glucocorticoid hormone 15-19 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

15-20 Figure 15.6 Heat shock protein
Heat shock proteins leave when hormone binds to receptor Nuclear localization Sequence is exposed Formation of a homodimer Glucocorticoid Response Elements These function as enhancers Transcription of target gene is activated Figure 15.6 15-20 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

The CREB Protein The CREB protein is another regulatory transcriptional factor functioning within living cells CREB is an acronym for cAMP response element-binding CREB protein becomes activated in response to cell-signaling molecules that cause an increase in cAMP Cyclic adenosine monophosphate The CREB protein recognizes a response element with the consensus sequence 5’–TGACGTCA–3’ This has been termed a cAMP response element (CRE) 15-21 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

15-22 Figure 15.7 The activity of the CREB protein
Could be a hormone, neurotransmitter, growth factor, etc. Acts as a second messenger Activates protein kinase A Phosphorylated CREB binds to DNA and stimulates transcription Unphosphorylated CREB can bind to DNA, but cannot activate RNA pol The activity of the CREB protein Figure 15.7 15-22

15.2 CHANGES IN CHROMATIN STRUCTURE
Changes in chromatin structure can involve changes in the structure of DNA and/or changes in chromosomal compaction These changes include 1. Gene amplification 2. Gene rearrangement 3. DNA methylation 4. Chromatin compaction Uncommon ways to regulate gene expression Common ways to regulate gene expression Refer to Table 15.1 15-23 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Chromatin Structure The three-dimensional packing of chromatin is an important parameter affecting gene expression Chromatin is a very dynamic structure that can alternate between two conformations Closed conformation Chromatin is very tightly packed Transcription may be difficult or impossible Open conformation Chromatin is highly extended Transcription can take place 15-25 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Figure 15.8 shows micrographs of a chromosome from an amphibian oocyte
Variations in the degree of chromatin packing occur in eukaryotic chromosomes during interphase During gene activation, tightly packed chromatin must be converted to an open conformation In order for transcription to occur Figure 15.8 shows micrographs of a chromosome from an amphibian oocyte The chromosome does not form a uniform 30 nm fiber Instead many decondensed loops radiate outward These are DNA regions whose genes are actively transcribed These chromosomes have been named lampbrush chromosomes They resemble brushes once used to clean kerosene lamps 15-26 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Experiment 15A: DNase I Sensitivity and Chromatin Structure
DNase I is an endonuclease that cleaves DNA It is much more likely to cleave DNA in an open conformation than in a closed conformation In 1976, Harold Weintraub and Mark Groudine used DNase I sensitivity to study chromatin structure In particular, they focused attention on the b-globin gene The gene was known to be specifically expressed in reticulocytes (immature red blood cells) But not in other cell types, such as brain cells and fibroblasts 15-27 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

First, let’s consider the rationale behind Weintraub and Groudine’s experimental approach
Globin genes are only a small part of the total DNA Therefore, they had to find a way to specifically monitor the digestion of the b-globin gene They used a radiolabeled cloned DNA fragment (i.e., a probe) that was complementary to the b-globin gene This was hybridized to the chromosomal DNA to determine specifically if the chromosomal b-globin gene was intact Following hybridization, the samples were then exposed to another enzyme, termed S1 nuclease This enzyme only cuts single-stranded DNA Refer to Figure 15.9 15-28 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Figure 15.9 Cut with DNase I Do not cut with DNase I This indicates that the chromosomal DNA was in an open conformation This indicates that the chromosomal DNA was in a closed conformation It was accessible to DNase I and was consequently digested It was inaccessible to DNase I and was thus protected from digestion 15-29

Testing the Hypothesis
A loosening of chromatin structure occurs when globin genes are transcriptionally active Testing the Hypothesis Refer to Figure 15.10 15-30 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Figure 15.10 15-31

Figure 15.10 15-32

Figure 15.10 15-33

% Hybridization of DNA probe
The Data Source of nuclei % Hybridization of DNA probe Reticulocytes 25% Brain cells >94% Fibrobalsts 15-34 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

% Hybridization of DNA probe
Interpreting the Data Source of nuclei % Hybridization of DNA probe Reticulocytes 25% Brain cells >94% Fibrobalsts Reticulocytes had a much smaller percentage of hybridization Therefore, their globin genes were more sensitive to DNase I The globin genes are known to be expressed in reticulocytes but not in brain cells and fibroblasts Therefore, these results are consistent with the hypothesis: The globin gene is less tightly packed when it is being expressed 15-35 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Globin Gene Expression
The family of globin genes is expressed in the reticulocytes However, individual members are expressed at different stages of development For example: b-globin  Adult g-globin  Fetus As shown in Figure 15.11a, several of the globin genes are adjacent to each other on chromosome 11 15-36 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Segments of DNA that are deleted in these populations
Figure 15.11 Segments of DNA that are deleted in these populations Thalassemia is a defect in the expression of one or more globin genes An intriguing observation of some thalassemic patients is that they cannot synthesize b-globin even though the gene is perfectly normal As shown in Figure 15.11b, this type of thalassemia involves a DNA deletion that occurs upstream of the b-globin gene The b-globin gene is intact However, it is turned off in these patients 15-37 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Genes are now accessible to RNA pol and transcription factors
A DNA region upstream of the b-globin gene was identified as necessary for globin gene expression This region is termed locus control region (LCR) It helps in the regulation of chromatin opening and closing It is missing in certain persons with thalassemias Genes are now accessible to RNA pol and transcription factors Figure 15.12 15-38 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Globin Gene Expression
Aside from chromatin packing, a second structural issue to consider is the position of nucleosomes In chromatin, the nucleosomes are usually positioned at regular intervals along the DNA However, they have been shown to change positions in cells that normally express a particular gene But not in cells where the gene is inactive Refer to Figure 15.13 15-39 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Positioned at regular intervals from -3,000 to + 1,500
Disruption in nucleosome positioning from -500 to + 200 Changes in nucleosome position during the activation of the b-globin gene Figure 15.3 15-40 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Chromatin Remodeling As discussed in Chapter 12, there are two common ways in which chromatin structure is altered 1. Covalent modification of histones 2. ATP-dependent chromatin remodeling So let’s review Figure 12.13 15-41 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Removes acetyl groups, thereby restoring a tighter interaction
1. Covalent modification of histones Amino terminals of histones are modified in various ways Acetylation; phosphorylation; methylation Adds acetyl groups, thereby loosening the interaction between histones and DNA Removes acetyl groups, thereby restoring a tighter interaction Figure 12.13 15-42 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

2. ATP-dependent chromatin remodeling
The energy of ATP is used to alter the structure of nucleosomes and thus make the DNA more accessible Proteins are members of the SWI/SNF family Acronyms refer to the effects on yeast when these enzyme are defective Mutants in SWI are defective in mating type switching Mutants in SNF are sucrose non-fermenters Figure 12.13 These effects may significantly alter gene expression 15-43 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Chromatin Remodeling An important role for transcriptional activators is to recruit the aforementioned enzymes to the promoter A well-studied example of recruitment involves a gene in yeast that is involved in mating Yeast can exist in two mating types, termed a and a The gene HO encodes an enzyme that is required for the mating switch Refer to Figure 15.14 15-44 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

15-45 Figure 15.14 SWI refers to mating type switching
SAGA is an acronym for Spt/Ada/GCN5/Acetyltransferase Genes known to be transcriptionally regulated by histone acetyltransferase Figure 15.14 15-45

SBP is an acronym for a mating type switching cell cycle box protein)
Figure 15.14 RNA polymerase 15-46

DNA Methylation DNA methylation is a change in chromatin structure that silences gene expression It is common in some eukaryotic species, but not all Yeast and Drosophila have little DNA methylation Vertebrates and plants have abundant DNA methylation In mammals, ~ 2 to 7% of the DNA is methylated Refer to Figure 15.15 15-47 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Only one strand is methylated Both strands are methylated
CH3 (or DNA methylase) Only one strand is methylated CH3 Both strands are methylated Figure 15.15 15-48

DNA methylation usually inhibits the transcription of eukaryotic genes
Especially when it occurs in the vicinity of the promoter In vertebrates and plants, many genes contain CpG islands near their promoters These CpG islands are 1,000 to 2,000 nucleotides long In housekeeping genes The CpG islands are unmethylated Genes tend to be expressed in most cell types In tissue-specific genes The expression of these genes may be silenced by the methylation of CpG islands 15-49 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

15-50 Figure 15.16 Transcriptional silencing via methylation
Transcriptional activator binds to unmethylated DNA This would inhibit the initiation of transcription Transcriptional silencing via methylation Figure 15.16 15-50

Transcriptional silencing via methylation
Figure 15.16 15-51

DNA Methylation is Heritable
Methylated DNA sequences are inherited during cell division Figure illustrates a model explaining how methylation is passed from mother to daughter cell 15-52 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

An infrequent and highly regulated event
Figure 15.17 An infrequent and highly regulated event CH3 CH3 Hemimethylated DNA Maintenance methylation CH3 DNA methylase converts hemi-methylated to fully- methylated DNA An efficient and routine event occurring in vertebrate and plant cells 15-53

15.3 REGULATION OF RNA PROCESSING AND TRANSLATION
So far, we have discussed various mechanisms that regulate the level of gene transcription In eukaryotic species, it is also common for gene expression to be regulated at the RNA level Refer to Table 15.2 15-54 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Alternative Splicing One very important biological advantage of introns in eukaryotes is the phenomenon of alternative splicing Alternative splicing refers to the fact that pre-mRNA can be spliced in more than one way In most cases, this produces two alternative versions of a protein that have similar functions Because much of their amino acid sequences are identical Nevertheless, there will be enough differences in amino acid sequences to provide each protein with its own characteristics 15-56 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Alternative Splicing The degree of splicing and alternative splicing varies greatly among different species Baker’s yeast contains about 6,000 genes ~ 300 (i.e., 5%) encode mRNAs that are spliced Only a few of these 300 have been shown to be alternatively spliced Humans contain ~ 35,000 genes Most of these encode mRNAs that are spliced It is estimated that a minimum of one-third of are alternatively spliced Note: Certain mRNAs can be alternatively spliced to produce dozens or even hundreds of different mRNAs 15-57 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Alternative Splicing Figure considers an example of alternative splicing for a gene that encodes a-tropomyosin This protein functions in the regulation of cell contraction It is found in Smooth muscle cells (uterus and small intestine) Striated muscle cells (cardiac and skeletal muscle) The different cells of a multicellular organism regulate their contraction in subtly different ways One way to accomplish this is to produce different forms of a-tropomyosin by alternative splicing 15-58 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Found in the mature mRNA from all cell types
Not found in all mature mRNAs These alternatively spliced versions of a-tropomyosin vary in function to meet the needs of the cell type in which they are found Figure Alternative ways that the rat a-tropomyosin pre-mRNA can be spliced 15-59

Alternative Splicing Alternative splicing is not a random event
The specific pattern of splicing is regulated in a given cell It involves proteins known as splicing factors These play a key role in the choice of splice sites One example of splicing factors is the SR proteins At their C-terminal end, they have a domain that is rich in serine (S) and arginine (R) It is involved in protein-protein recognition At their N-terminal end, they have an RNA-binding domain 15-60 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

This can occur in two ways
The spliceosome recognizes the 5’ and 3’ splice sites and removes the intervening intron Refer to Chapter 12 Splicing factors modulate the ability of spliceosomes to recognize or choose the splice sites This can occur in two ways 1. Some splicing factors inhibit the ability of a spliceosome to recognize a splice site Refer to Figure 15.19a 2. Some splicing factors enhance the ability of a spliceosome to recognize a splice site Refer to Figure 15.19b 15-61 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Figure 15.19 The role of splicing factors during alternative splicing

RNA Editing The term RNA editing refers to a change in the nucleotide sequence of an RNA molecule It involves additions or deletion of particular bases Or a conversion of one type of base to another RNA editing can have various effects on mRNAs Generating start or stop codons Changing the coding sequence of a polypeptide Table 15.3 describes several examples where RNA editing has been found 15-64 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

RNA Editing RNA editing was first discovered in trypanosomes
The protozoa that cause sleeping sickness In these organisms, the process involves guide RNA Guide RNA can direct the addition or deletion of one or more uracils into an RNA Refer to Figure 15.20 15-66 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

15-67 Figure 15.20 5’ end is complementary to mRNA being edited
3’ end has a sequence of uracils Cleaves target DNA at a defined location First, the 5’ anchor binds to target DNA 3’ end of guide RNA becomes displaced from target DNA Removes uracils Inserts uracils Rejoins the two DNA pieces Figure 15.20 15-67

Recognized as guanine during translation
A more widespread mechanism for RNA editing involves changes of one type of base to another This involves deamination of bases Recognized as guanine during translation Figure 15.21 15-68 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Stability of mRNA The stability of eukaryotic mRNA varies considerably
Several minutes to several days The stability of mRNA can be regulated so that its half-life is shortened or lengthened This will greatly influence the mRNA concentration And consequently gene expression Factors that can affect mRNA stability include 1. Length of the polyA tail 2. Destabilizing elements 15-69 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

1. Length of the polyA tail
Most newly made mRNA have a polyA tail that is about 200 nucleotides long It is recognized by polyA binding protein Which binds to the polyA tail and enhances stability As an mRNA ages, its polyA tail is shortened by the action of cellular nucleases The polyA-binding protein can no longer bind if the polyA tail is less than 10 to 30 adenosines long The mRNA will then be rapidly degraded by exo- and endonucleases 15-70 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

5’-untranslated region 3’-untranslated region
2. Destabilizing elements Found especially in mRNAs that have short half-lives These elements can be found anywhere on the mRNA However, they are most common at the 3’ end between the stop codon and the polyA tail AU-rich element Recognized and bound by cellular proteins These proteins influence mRNA degradation 5’-untranslated region 3’-untranslated region Figure 15.22 15-71 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Double-stranded RNA and Gene Silencing
Double-stranded RNA can silence the expression of certain genes This discovery was made from research in plants and the nematode Caenorhabditis elegans Using cloning techniques, it is possible to introduce cloned genes into the genomes of plants When cloned genes were introduced in multiple copies, the expression of the gene was often silenced This may be due to the formation of double-stranded RNA 15-72 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

This event will silence the expression of the cloned gene
PE This event will silence the expression of the cloned gene PE PC Transcription occurs from both promoters Thus sense and anti-sense strands are transcribed This produces complementary RNAs that will form a double stranded structure Gene insertion leading to the production of double-stranded RNA Figure 15.23 15-73 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Evidence for mRNA degradation via double-stranded RNA came from studies in C. elegans Injection of antisense RNA (i.e., RNA complementary to a specific mRNA) into oocytes silences gene expression Surprisingly, injection of double-stranded RNA was 10 times more potent at inhibiting the expression of the corresponding mRNA This phenomenon was termed RNA interference (RNAi) A proposed mechanism for RNAi is shown in Figure 15.24 15-74 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

15-75 Figure 15.24 Short RNA from the antisense strand
Thus the expression of the gene that encodes this mRNA is silenced Cellular mRNA is degraded by endonucleases within the complex Figure 15.24 15-75

RNA interference is widely found in eukaryotes It is believed to 1. Offer a host defense mechanism against certain viruses Those with double-stranded RNA genomes, in particular 2. Play a role in silencing certain transposable elements Some of these elements produce double-stranded RNA intermediates as part of the transposition process 15-76 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Initiation Factors and the Rate of Translation
Modulation of translation initiation factors is widely used to control fundamental cellular processes Under certain conditions, it is advantageous for a cell to stop synthesizing proteins Viral infection So that the virus cannot manufacture viral proteins Starvation So that the cell conserves resources 15-77 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

The phosphorylation of initiation factors has been found to affect translation in eukaryotic cells
Two initiation factors appear to play a central role in controlling the initiation of translation eIF2 and eIF4F The function of these two factors are modulated by phosphorylation in opposite ways Phosphorylation of eIF2a inhibits translation Phosphorylation of eIF4F increases the rate of translation Figure shows the events leading to the translational inhibition by eIF2a 15-78 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Required if eIF2 is to promote binding of the initiator tRNAmet to the 40S subunit

eIF4F provides another way to control translation
It regulates the binding of mRNA to the ribosomal initiation complex eIF4F is stimulated by phosphorylation Conditions that increase its phosphorylation include signaling molecules that promote cell proliferation Growth factors and insulin, for example Conditions that decrease its phosphorylation include heat shock and viral infection 15-80 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Iron Assimilation and Translation
Regulation of iron assimilation provides an example how the translation of specific mRNAs is modulated Iron is an essential element for the survival of living organisms It is required for the function of many different enzymes The assimilation of iron is depicted in Figure 15.26 15-81 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

15-82 Figure 15.26 Protein that carries iron through the bloodstream
A hollow spherical protein Prevents toxic buildup of too much iron in the cell Figure 15.26 15-82 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Iron is a vital yet potentially toxic substance
So mammalian cells have evolved an interesting way to regulate iron assimilation An RNA-binding protein known as the iron regulatory protein (IRP) plays a key role It influences both the ferritin mRNA and the transferrin receptor mRNA This protein binds to a regulatory element within the mRNA known as the iron response element (IRE) IRE is found in the 5’-UTR in ferritin mRNA And in the 3’-UTR in transferrin receptor mRNA Regulation of iron assimilation is shown in Figure 15.27 15-83 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

15-84 Figure 15.27 (a) Regulation of ferritin mRNA
Iron regulatory protein binds IRE This inhibits translation Iron regulatory protein binds iron It is released from IRE Translation of ferritin proceeds 15-84 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

More mRNA means more translation And enhances the stability of mRNA
Figure 15.27 (b) Regulation of transferrin receptor mRNA More mRNA means more translation IRP binds IRE And enhances the stability of mRNA IRP binds iron It is released from IRE mRNA rapidly degraded 15-85 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

PowerPoint Presentation Materials to accompany

Similar presentations

Presentation on theme: "PowerPoint Presentation Materials to accompany"— Presentation transcript:

Similar presentations

About project

Feedback

Log in

Auth with social network:

PowerPoint Presentation Materials to accompany

Similar presentations

Presentation on theme: "PowerPoint Presentation Materials to accompany"— Presentation transcript:

Similar presentations

About project

Feedback