Slide 1 Chapter 18 Regulation of Gene Expression PowerPoint Lecture Presentations for Biology Eighth Edition Neil Campbell and Jane Reece Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Slide 2 Changes in the environment can regulate cell activity Natural selection has favored single-celled organisms that produce only the products needed by that cell The fastest way is to directly control enzyme cascades In bacteria it is nearly as fast to change gene expression. This is controlled by the operon model one promoter, many genes Eukaryotes also have coordinately controlled gene expression but, like everything else about them it s more complicated (more on that later...) Slide 3 Fig. 18-2 Precursor trpe gene Feedback Loop: Enzyme activity directly controlled by presence or absence of substrate Enzyme 1 Enzyme 2 Enzyme 3 Tryptophan trpd gene trpc gene trpb gene trpa gene Operon: Gene expression in a cluster of functionally related genes can be coordinated by a single on-off switch
Slide 4 Operons: the entire stretch of that includes... The promoter which binds polymerase. The operator: a regulatory switch in the promoter that reacts to environmental change All of the related genes that they control Slide 5 Negative vs. Positive Operons Two Types of Negative, or Repressor, Gene Regulation Operons that involve negative control of genes are switched off by the active form of a repressor A repressible operon is one that is usually on; binding of a repressor to the operator shuts off transcription An inducible operon is one that is usually off; a molecule called an inducer inactivates the repressor and turns on transcription One Type of Positive, or Activator, Gene Regulation An activator of transcription is a stimulatory protein that acts to bind polymerase much like an eukaryotic transcription factor Slide 6 Example 1 of Negative, or Repressor, Gene Regulation Repressible enzymes usually function in anabolic pathways; their synthesis is repressed by high levels of the end product The trp operon is a repressible operon: By default it is on and the genes for tryptophan synthesis are transcribed Tryptophan is a corepressor and when it is present, it binds to the trp repressor protein, which turns the operon off
Slide 7 Fig. 18-3a trp operon Genes of operon trpr trpe trpd trpc trpb trpa Regulatory Operator gene Start codon Stop codon 3 5 5 polymerase E D C B A Protein Inactive Polypeptide subunits that make up repressor enzymes for tryptophan synthesis Tryptophan absent, repressor inactive, operon on Slide 8 Fig. 18-3b-1 No made Protein Tryptophan (corepressor) Active repressor Tryptophan present, repressor active, operon off Slide 9 Fig. 18-3b-2 No made Protein Tryptophan (corepressor) Active repressor Tryptophan present, repressor active, operon off
Slide 10 Example 2 of Negative, or Repressor, Gene Regulation Inducible enzymes usually function in catabolic pathways; their synthesis is induced by a chemical signal The lac operon is an inducible operon and by itself, the lac repressor is active and switches the lac operon off Lactose is an inducer and inactivates the repressor to turn the lac operon on Slide 11 Fig. 18-4a Regulatory gene 5 laci 3 polymerase Operator lacz No made Protein Active repressor Lactose absent, repressor active, operon off Slide 12 Fig. 18-4b laci lac operon lacz lacy laca 5 Protein 3 polymerase 5 β-galactosidase Permease Transacetylase Allolactose Inactive (inducer) repressor Lactose present, repressor inactive, operon on
Slide 13 Positive Gene Regulation Positive secondary control of the speed of an operon Sometimes lactose is present but glucose is scarce Catabolite Activator Protein (CAP) is activated by binding with cyclic AMP Activated CAP attaches to the promoter of the lac operon and increases the affinity of polymerase, thus accelerating transcription When glucose levels increase, CAP detaches from the lac operon, and transcription returns to a normal rate CAP helps regulate other operons that encode enzymes used in catabolic pathways Slide 14 Fig. 18-5 Operator laci lacz Lactose present, glucose scarce (camp level high): abundant lac synthesized CAP-binding site camp Inactive CAP Active CAP polymerase binds and transcribes Inactive lac repressor Lactose present, glucose present (camp level low): less lac synthesized laci CAP-binding site Inactive CAP Operator lacz polymerase less likely to bind Inactive lac repressor Slide 15 Differential gene expression in multicellular development and cell specialization Regulation of Chromatin Structure Regulation of Transcription Initiation Mechanisms of Post-Transcriptional Regulation Mechanisms of Post-Translational Regulation
Slide 16 Fig. 18-6a Signal Chromatin NUCLEUS Cap Chromatin modification Gene available for transcription Gene Transcription Primary transcript Intron processing Tail in nucleus Transport to cytoplasm CYTOPLASM Slide 17 Fig. 18-6b CYTOPLASM in cytoplasm Degradation of Translation Polypeptide Degradation of protein Protein processing Active protein Transport to cellular destination Cellular function Slide 18 Regulation of Chromatin Structure Genes within highly packed heterochromatin are usually not expressed Chemical modifications to histones and influence chromatin structure and gene expression by making a region of either more or less able to bind the transcription machinery The histone code hypothesis proposes that specific combinations of modifications help determine chromatin configuration and influence transcription
Slide 19 Fig. 18-7 In histone acetylation, acetyl groups are attached to positively charged lysines in histone tails This process loosens chromatin structure, thereby initiating transcription double helix (a) Histone tails protrude outward from a nucleosome Unacetylated histones Histone tails Amino acids available for chemical modification Acetylated histones The addition of methyl groups (methylation) can condense chromatin The addition of phosphate groups (phosphorylation) next to a methylated amino acid can loosen chromatin Slide 20 Methylation methylation, the addition of methyl groups to certain bases in, is associated with reduced transcription in some species 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 Slide 21 Epigenetic Inheritance Although the chromatin modifications just discussed do not alter sequence, they may be passed to future generations of cells The inheritance of traits transmitted by mechanisms not directly involving the nucleotide sequence is called epigenetic inheritance
Slide 22 Fig. 18-8-1 Organization of a Typical Eukaryotic Gene Enhancer (distal control elements) Upstream Proximal control elements Intron Intron Poly-A signal sequence Termination region Downstream Slide 23 Fig. 18-8-2 Organization of a Typical Eukaryotic Gene Enhancer (distal control elements) Upstream Proximal control elements Intron Intron Transcription Poly-A signal sequence Termination region Downstream Primary transcript 5 Intron Intron Poly-A signal Cleaved 3 end of primary transcript Slide 24 Fig. 18-8-3 Organization of a Typical Eukaryotic Gene Enhancer (distal control elements) Upstream Proximal control elements Intron Intron Transcription Poly-A signal sequence Termination region Downstream Primary transcript 5 Intron 5 Cap 5 UTR Intron Intron Coding segment Start codon processing Poly-A signal Stop codon 3 UTR Poly-A tail What do the GTP Cap and Poly-A Tail do again? Cleaved 3 end of primary transcript 3
Slide 25 Fig. 18-9-1 Activators Enhancer Distal control element TATA box Gene Slide 26 Fig. 18-9-2 Activators Enhancer Distal control element TATA box Gene -bending protein General transcription factors Tissue-specific transcription factors Slide 27 Fig. 18-9-3 Activators Enhancer Distal control element TATA box Gene -bending protein General transcription factors Tissue-specific transcription factors polymerase II polymerase II Transcription initiation complex synthesis
Slide 28 Fig. 18-10 A particular combination of control elements can activate transcription only when the appropriate activator proteins and transcription factors are present Control elements Available proteins (a) Liver cell Enhancer LIVER CELL NUCLEUS Albumin gene expressed Crystallin gene not expressed Albumin gene (b) Lens cell Crystallin gene LENS CELL NUCLEUS Available proteins Albumin gene not expressed Crystallin gene expressed Unlike prokaryotes, coordinately controlled eukaryotic genes can be scattered over different chromosomes, but each has the same combination of control elements Slide 29 Fig. 18-11 s Troponin T gene Primary transcript splicing or Slide 30 Fig. 18-12 Proteasomes are giant protein complexes that bind protein molecules and degrade them Ubiquitin Proteasome Proteasome and ubiquitin to be recycled Protein to be degraded Ubiquitinated protein Protein entering a proteasome Protein fragments (peptides) After translation, various types of protein processing, including cleavage and the addition of chemical groups, are subject to control
Slide 31 Noncoding s can control gene expression through Interference (i). Only a small fraction of codes for proteins, r, and t. A significant amount of the genome may be transcribed into noncoding s Micros (mis) are small single-stranded molecules that can bind to and degrade it or block its translation sis and mis are similar but form from different precursors sis play a role in heterochromatin formation and can block large regions of the chromosome Small s may also block transcription of specific genes Slide 32 Fig. 18-UN5 Chromatin modification Transcription processing Chromatin modification Small s can promote the formation of heterochromatin in certain regions, blocking transcription. Translation mi or si can block the translation of specific s. degradation Translation Protein processing and degradation degradation mi or si can target specific s for destruction. Slide 33 Fig. 18-13 Hairpin mi Hydrogen bond Dicer 5 3 (a) Primary mi transcript mi miprotein complex degraded Translation blocked (b) Generation and function of mis
Slide 34 Fig. 18 14 The differentiation of cell types in a multicellular organism results from orchestrated changes in gene expression Fertilized eggs are single cells This tadpole has neurons, muscle, gills, fins, etc. Slide 35 Fig. 18 15a An egg s cytoplasm contains, proteins, and other substances that are distributed unevenly in the unfertilized egg Sperm Fertilization Mitotic cell division Unfertilized egg cell Two different cytoplasmic determinants Zygote Nucleus Two celled embryos have two different cell types Slide 36 Fig. 18 15b Early embryo (32 cells) In the process called induction, signal molecules from nearby cells cause transcriptional changes in embryonic target cells NUCLEUS Signal transduction pathway Signal receptor Signal molecule (inducer)
Slide 37 Fig. 18 17b 1 Egg cell developing within ovarian follicle Follicle cell Nucleus Egg cell Nurse cell Ovary cells can tell the unfertilized egg which end is which 2 Unfertilized egg Depleted nurse cells 3 Fertilized egg Egg shell Fertilization Laying of egg Embryonic development 4 Segmented embryo 5 Larval stage Body segments Hatching 0.1 mm Slide 38 Fig. 18 19c Bicoid: Nurse cells help localize the for a protein that is required to make head structures Nurse cells Egg bicoid Developing egg Bicoid in mature unfertilized egg Bicoid protein in early embryo Slide 39 Fig. 18 19a EXPERIMENT Tail Head T1 T2 T3 A1 A2 A3 A4 A5 A6 A7 A8 Tail Wild type larva Tail A8 Mutant larva (bicoid) A7 A6 A7 A8
Slide 40 Fig. 18 16 1 Nucleus Master regulatory gene myod Other muscle specific genes Embryonic precursor cell Slide 41 Fig. 18 16 2 Nucleus Master regulatory gene myod Other muscle specific genes Embryonic precursor cell Myoblast (determined) MyoD protein (transcription factor) Slide 42 Fig. 18 16 3 Nucleus Master regulatory gene myod Other muscle specific genes Embryonic precursor cell Myoblast (determined) MyoD protein (transcription factor) Part of a muscle fiber (fully differentiated cell) MyoD Another transcription factor Myosin, other muscle proteins, and cell cycle blocking proteins