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Chapter 18 Regulation of Gene Expression
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Overview: Conducting the Genetic Orchestra
Prokaryotes and eukaryotes alter gene expression in response to their changing environment In multicellular eukaryotes, gene expression regulates development and is responsible for differences in cell types RNA molecules play many roles in regulating gene expression in eukaryotes © 2011 Pearson Education, Inc.
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Concept 18.1: Bacteria respond to environmental change by regulating transcription
Natural selection favors bacteria that produce only the products needed by that cell Regulation feedback inhibition gene regulation Gene expression in bacteria is controlled by the operon model © 2011 Pearson Education, Inc.
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Regulation of gene expression
Figure 18.2 Precursor Feedback inhibition trpE gene Enzyme 1 trpD gene Regulation of gene expression Enzyme 2 trpC gene trpB gene Figure 18.2 Regulation of a metabolic pathway. Enzyme 3 trpA gene Tryptophan (a) Regulation of enzyme activity (b) Regulation of enzyme production
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Operons: The Basic Concept
An operon -entire stretch of DNA that includes the operator, the promoter, and the genes that they control A cluster of functionally related genes can be under coordinated control by a single “on-off switch” “switch” is a segment of DNA called an operator © 2011 Pearson Education, Inc.
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The operon can be switched off by a protein repressor
prevents gene transcription by binding to the operator and blocking RNA polymerase Made by separate regulatory gene © 2011 Pearson Education, Inc.
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Operon Model The repressor can be in an active or inactive form, depending on the presence of other molecules A corepressor is a molecule that cooperates with a repressor protein to switch an operon off For example, E. coli can synthesize the amino acid tryptophan © 2011 Pearson Education, Inc.
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Tryptophan Operon trp operon is on unless switched off by repressor
Anabolic- tryptophan needed When tryptophan is present, it binds to the trp repressor protein, which turns the operon off thus the trp operon is turned off (repressed) if tryptophan levels are high (saves cell energy) © 2011 Pearson Education, Inc.
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Polypeptide subunits that make up enzymes for tryptophan synthesis
Figure 18.3a trp operon Promoter Promoter Genes of operon DNA trpR trpE trpD trpC trpB trpA Operator Regulatory gene RNA polymerase Start codon Stop codon 3 mRNA 5 mRNA 5 E D C B A Protein Inactive repressor Figure 18.3 The trp operon in E. coli: regulated synthesis of repressible enzymes. Polypeptide subunits that make up enzymes for tryptophan synthesis (a) Tryptophan absent, repressor inactive, operon on
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Tryptophan (corepressor)
Figure 18.3b-1 DNA mRNA Protein Active repressor Figure 18.3 The trp operon in E. coli: regulated synthesis of repressible enzymes. Tryptophan (corepressor) (b) Tryptophan present, repressor active, operon off
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Tryptophan (corepressor)
Figure 18.3b-2 DNA No RNA made mRNA Protein Active repressor Figure 18.3 The trp operon in E. coli: regulated synthesis of repressible enzymes. Tryptophan (corepressor) (b) Tryptophan present, repressor active, operon off
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lac Operon Inducible operon -usually off; a molecule called an inducer inactivates the repressor and turns on transcription Lactose absent- repressor is active- no enzyme produced Lactose present- inducer (allolactose) binds to repressor- allosteric interaction- operon is free to work © 2011 Pearson Education, Inc.
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(a) Lactose absent, repressor active, operon off
Figure 18.4a Regulatory gene Promoter Operator DNA DNA lacI lacZ No RNA made 3 mRNA RNA polymerase 5 Figure 18.4 The lac operon in E. coli: regulated synthesis of inducible enzymes. Active repressor Protein (a) Lactose absent, repressor active, operon off
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Allolactose (inducer)
Figure 18.4b lac operon DNA lacI lacZ lacY lacA RNA polymerase 3 mRNA mRNA 5 5 -Galactosidase Permease Transacetylase Protein Figure 18.4 The lac operon in E. coli: regulated synthesis of inducible enzymes. Inactive repressor Allolactose (inducer) (b) Lactose present, repressor inactive, operon on
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Repressible vs. Inducible
Inducible enzymes Repressible enzymes usually function in catabolic pathways; Normally off their synthesis is induced by a chemical signal i.e. lactose lac operon usually function in anabolic pathways; Normally on repressed by high levels of the end product i.e. tryptophan trp operon © 2011 Pearson Education, Inc.
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Negative Gene Regulation
Operon is off with active for of a repressor
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Positive Gene Regulation
Some operons are also subject to positive control through a stimulatory protein Ex. catabolite activator protein (CAP), an activator of transcription When glucose (a preferred food source of E. coli) is scarce, CAP is activated by binding with cyclic AMP (cAMP) Activated CAP attaches to the promoter of the lac operon and increases the affinity of RNA polymerase, thus accelerating transcription © 2011 Pearson Education, Inc.
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RNA polymerase binds and transcribes Operator
Figure 18.5a Promoter DNA lacI lacZ CAP-binding site RNA polymerase binds and transcribes Operator Active CAP cAMP Figure 18.5 Positive control of the lac operon by catabolite activator protein (CAP). Inactive lac repressor Inactive CAP Allolactose (a) Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized
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RNA polymerase less likely to bind
Figure 18.5b Promoter DNA lacI lacZ Operator CAP-binding site RNA polymerase less likely to bind Inactive CAP Inactive lac repressor Figure 18.5 Positive control of the lac operon by catabolite activator protein (CAP). (b) Lactose present, glucose present (cAMP level low): little lac mRNA synthesized
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Concept 18.2: Eukaryotic gene expression is regulated at many stages
In multicellular organisms regulation of gene expression is essential for cell specialization differential gene expression, the expression of different genes by cells with the same genome Results in different cell types © 2011 Pearson Education, Inc.
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Figure 18.6 Signal NUCLEUS Chromatin Chromatin modification: DNA unpacking involving histone acetylation and DNA demethylation DNA Gene available for transcription Gene Transcription RNA Exon Primary transcript Intron RNA processing Tail Cap mRNA in nucleus Transport to cytoplasm CYTOPLASM mRNA in cytoplasm Degradation of mRNA Translation Figure 18.6 Stages in gene expression that can be regulated in eukaryotic cells. Polypeptide Protein processing, such as cleavage and chemical modification Active protein Degradation of protein Transport to cellular destination Cellular function (such as enzymatic activity, structural support)
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Regulation of Chromatin Structure
Genes within highly packed heterochromatin are usually not expressed Chemical modifications to histones histone acetylation, acetyl groups are attached to positively charged lysines in histone tails loosens chromatin structure, promotes the initiation of transcription addition of methyl groups (methylation) can condense chromatin; and DNA of chromatin influence both chromatin structure and gene expression © 2011 Pearson Education, Inc.
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Amino acids available for chemical modification
Figure 18.7 Histone tails DNA double helix Amino acids available for chemical modification Nucleosome (end view) (a) Histone tails protrude outward from a nucleosome Figure 18.7 A simple model of histone tails and the effect of histone acetylation. Unacetylated histones Acetylated histones (b) Acetylation of histone tails promotes loose chromatin structure that permits transcription
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DNA Methylation DNA methylation, the addition of methyl groups to certain bases in DNA, is associated with reduced transcription in some species DNA methylation can cause long-term inactivation of genes in cellular differentiation In genomic imprinting, methylation regulates expression of either the maternal or paternal alleles of certain genes at the start of development © 2011 Pearson Education, Inc.
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Enhancer for liver-specific genes Enhancer for lens-specific genes
Figure 18.UN04a Chromatin modification Transcription • Genes in highly compacted chromatin are generally not transcribed. • Regulation of transcription initiation: DNA control elements in enhancers bind specific transcription factors. • Histone acetylation seems to loosen chromatin structure, enhancing transcription. Bending of the DNA enables activators to contact proteins at the promoter, initiating transcription. • DNA methylation generally reduces transcription. • Coordinate regulation: Enhancer for liver-specific genes Enhancer for lens-specific genes Chromatin modification Figure 18.UN04a Summary figure, Concept 18.2 (part 1) Transcription RNA processing • Alternative RNA splicing: RNA processing Primary RNA transcript mRNA degradation Translation mRNA or Protein processing and degradation
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Regulation of Transcription Initiation
Chromatin-modifying enzymes provide initial control of gene expression by making a region of DNA either more or less able to bind the transcription machinery Associated with most eukaryotic genes are multiple control elements, segments of noncoding DNA that serve as binding sites for transcription factors that help regulate transcription Control elements and the transcription factors they bind are critical to the precise regulation of gene expression in different cell types © 2011 Pearson Education, Inc.
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Enhancer (distal control elements) Proximal control elements
Figure Enhancer (distal control elements) Proximal control elements Poly-A signal sequence Transcription start site Transcription termination region DNA Exon Intron Exon Intron Exon Upstream Downstream Promoter Transcription Poly-A signal Primary RNA transcript (pre-mRNA) Exon Intron Exon Intron Exon Cleaved 3 end of primary transcript 5 RNA processing Intron RNA Figure 18.8 A eukaryotic gene and its transcript. Coding segment mRNA G P P P AAA AAA 3 Start codon Stop codon 5 Cap 5 UTR 3 UTR Poly-A tail
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Enhancers and Specific Transcription Factors
Proximal control elements are located close to the promoter Distal control elements, groupings of which are called enhancers, may be far away from a gene or even located in an intron An activator is a protein that binds to an enhancer and stimulates transcription of a gene © 2011 Pearson Education, Inc.
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Distal control element Enhancer TATA box General transcription factors
Figure Promoter Activators Gene DNA Distal control element Enhancer TATA box General transcription factors DNA- bending protein Group of mediator proteins RNA polymerase II Figure A model for the action of enhancers and transcription activators. RNA polymerase II Transcription initiation complex RNA synthesis
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Coordinately Controlled Genes in Eukaryotes
Unlike the genes of a prokaryotic operon, each of the co-expressed eukaryotic genes has a promoter and control elements These genes can be scattered over different chromosomes, but each has the same combination of control elements Copies of the activators recognize specific control elements and promote simultaneous transcription of the genes © 2011 Pearson Education, Inc.
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Protein processing and degradation
Figure 18.UN04b Chromatin modification Transcription RNA processing mRNA degradation Translation Protein processing and degradation Translation • Initiation of translation can be controlled via regulation of initiation factors. Figure 18.UN04b Summary figure, Concept 18.2 (part 2) mRNA degradation • Each mRNA has a characteristic life span, determined in part by sequences in the 5 and 3 UTRs. Protein processing and degradation • Protein processing and degradation by proteasomes are subject to regulation.
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Mechanisms of Post-Transcriptional Regulation
RNA processing- alternative RNA splicing, regulatory proteins determine what is removed mRNA degradation- can get translated repeatedly Regulation of the initiation of translation Most common method for regulation of gene expression translation of all mRNAs in a cell may be regulated simultaneously from signal from cell communication © 2011 Pearson Education, Inc.
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Primary RNA transcript
Figure 18.13 Exons DNA 1 2 3 4 5 Troponin T gene Primary RNA transcript 1 2 3 4 5 Figure Alternative RNA splicing of the troponin T gene. RNA splicing mRNA or 1 2 3 5 1 2 4 5
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Protein Processing and Degradation
After translation, various types of protein processing, including cleavage and the addition of chemical groups, are subject to control Proteasomes are giant protein complexes that bind protein molecules and degrade them © 2011 Pearson Education, Inc.
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Proteasome and ubiquitin to be recycled Ubiquitin
Figure 18.14 Proteasome and ubiquitin to be recycled Ubiquitin Proteasome Protein to be degraded Ubiquitinated protein Protein fragments (peptides) Protein entering a proteasome Figure Degradation of a protein by a proteasome.
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© 2011 Pearson Education, Inc.
Concept 18.3: Noncoding RNAs play multiple roles in controlling gene expression A significant amount of the genome may be transcribed into noncoding RNAs (ncRNAs) Noncoding RNAs regulate gene expression at two points: mRNA translation and chromatin configuration © 2011 Pearson Education, Inc.
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Effects on mRNAs by MicroRNAs and Small Interfering RNAs
MicroRNAs (miRNAs) are small single-stranded RNA molecules that can bind to mRNA These can degrade mRNA or block its translation © 2011 Pearson Education, Inc.
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(a) Primary miRNA transcript miRNA miRNA- protein complex
Figure 18.15 Hairpin Hydrogen bond miRNA Dicer 5 3 (a) Primary miRNA transcript miRNA miRNA- protein complex Figure Regulation of gene expression by miRNAs. mRNA degraded Translation blocked (b) Generation and function of miRNAs
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© 2011 Pearson Education, Inc.
The phenomenon of inhibition of gene expression by RNA molecules is called RNA interference (RNAi) RNAi is caused by small interfering RNAs (siRNAs) siRNAs and miRNAs are similar but form from different RNA precursors © 2011 Pearson Education, Inc.
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© 2011 Pearson Education, Inc.
Concept 18.4: A program of differential gene expression leads to the different cell types in a multicellular organism During embryonic development, a fertilized egg gives rise to many different cell types Cell types are organized successively into tissues, organs, organ systems, and the whole organism Gene expression orchestrates the developmental programs of animals © 2011 Pearson Education, Inc.
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A Genetic Program for Embryonic Development
The transformation from zygote to adult results from cell division, cell differentiation, and morphogenesis Cell differentiation is the process by which cells become specialized in structure and function The physical processes that give an organism its shape constitute morphogenesis © 2011 Pearson Education, Inc.
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Cytoplasmic Determinants and Inductive Signals
An egg’s cytoplasm contains RNA, proteins, and other substances that are distributed unevenly in the unfertilized egg Cytoplasmic determinants are maternal substances in the egg that influence early development © 2011 Pearson Education, Inc.
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(a) Cytoplasmic determinants in the egg
Figure 18.17a (a) Cytoplasmic determinants in the egg Unfertilized egg Sperm Nucleus Fertilization Molecules of two different cytoplasmic determinants Zygote (fertilized egg) Figure Sources of developmental information for the early embryo. Mitotic cell division Two-celled embryo
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© 2011 Pearson Education, Inc.
In the process called induction, signal molecules from embryonic cells cause transcriptional changes in nearby target cells Thus, interactions between cells induce differentiation of specialized cell types © 2011 Pearson Education, Inc.
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(b) Induction by nearby cells
Figure 18.17b (b) Induction by nearby cells Early embryo (32 cells) NUCLEUS Signal transduction pathway Figure Sources of developmental information for the early embryo. Signal receptor Signaling molecule (inducer)
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© 2011 Pearson Education, Inc.
Sequential Regulation of Gene Expression During Cellular Differentiation Determination commits a cell to its final fate Determination precedes differentiation Cell differentiation is marked by the production of tissue-specific proteins Ex- muscle cell- regulatory gene commits cell to being muscle- makes myo D transcription factor to bind to enhancers and stimulates expression © 2011 Pearson Education, Inc.
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Master regulatory gene myoD
Figure Nucleus Master regulatory gene myoD Other muscle-specific genes DNA Embryonic precursor cell OFF OFF mRNA OFF MyoD protein (transcription factor) Myoblast (determined) Figure Determination and differentiation of muscle cells. mRNA mRNA mRNA mRNA Myosin, other muscle proteins, and cell cycle– blocking proteins MyoD Another transcription factor Part of a muscle fiber (fully differentiated cell)
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Pattern Formation: Setting Up the Body Plan
Pattern formation is the development of a spatial organization of tissues and organs In animals, pattern formation begins with the establishment of the major axes Levels of morphogens establish an embryo’s axes and other features Positional information, the molecular cues that control pattern formation, tells a cell its location relative to the body axes and to neighboring cells Studied extensively with Drosophila- used mutants © 2011 Pearson Education, Inc.
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© 2011 Pearson Education, Inc.
Concept 18.5: Cancer results from genetic changes that affect cell cycle control The gene regulation systems that go wrong during cancer are the very same systems involved in embryonic development © 2011 Pearson Education, Inc.
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Types of Genes Associated with Cancer
Cancer can be caused by mutations to genes that regulate cell growth and division Oncogenes are cancer-causing genes Proto-oncogenes are the corresponding normal cellular genes responsible for normal cell growth and division Conversion of a proto-oncogene to an oncogene can lead to abnormal stimulation of the cell cycle Tumor viruses can cause cancer in animals including humans © 2011 Pearson Education, Inc.
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within a control element
Figure 18.23 Proto-oncogene DNA Translocation or transposition: gene moved to new locus, under new controls Gene amplification: multiple copies of the gene Point mutation: within a control element within the gene New promoter Oncogene Oncogene Figure Genetic changes that can turn proto-oncogenes into oncogenes. Normal growth- stimulating protein in excess Normal growth-stimulating protein in excess Normal growth- stimulating protein in excess Hyperactive or degradation- resistant protein
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© 2011 Pearson Education, Inc.
Proto-oncogenes can be converted to oncogenes by Movement of DNA within the genome: if it ends up near an active promoter, transcription may increase Amplification of a proto-oncogene: increases the number of copies of the gene Point mutations in the proto-oncogene or its control elements: cause an increase in gene expression © 2011 Pearson Education, Inc.
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Tumor-Suppressor Genes
Tumor-suppressor genes help prevent uncontrolled cell growth Mutations that decrease protein products of tumor-suppressor genes may contribute to cancer onset Tumor-suppressor proteins Repair damaged DNA Control cell adhesion Inhibit the cell cycle in the cell-signaling pathway © 2011 Pearson Education, Inc.
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Protein overexpressed Protein absent
Figure 18.24c EFFECTS OF MUTATIONS Protein overexpressed Protein absent Cell cycle overstimulated Increased cell division Cell cycle not inhibited Figure Signaling pathways that regulate cell division. (c) Effects of mutations
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Figure 18.UN04 Summary figure, Concept 18.2
Chromatin modification Transcription • Genes in highly compacted chromatin are generally not transcribed. • Regulation of transcription initiation: DNA control elements in enhancers bind specific transcription factors. • Histone acetylation seems to loosen chromatin structure, enhancing transcription. Bending of the DNA enables activators to contact proteins at the promoter, initiating transcription. • DNA methylation generally reduces transcription. • Coordinate regulation: Enhancer for liver-specific genes Enhancer for lens-specific genes Chromatin modification Transcription RNA processing • Alternative RNA splicing: RNA processing Primary RNA transcript mRNA degradation Translation mRNA or Figure 18.UN04 Summary figure, Concept 18.2 Protein processing and degradation Translation • Initiation of translation can be controlled via regulation of initiation factors. mRNA degradation • Each mRNA has a characteristic life span, determined in part by sequences in the 5 and 3 UTRs. Protein processing and degradation • Protein processing and degradation by proteasomes are subject to regulation.
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