Chapter 18_Bacterial gene regulation_upload.pptx

Biological Science Seventh Edition Chapter 18 Control of Gene Expression in Bacteria Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Chapter 18 Opening Roadmap Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Introduction to Control of Gene Expression...

Biological Science Seventh Edition Chapter 18 Control of Gene Expression in Bacteria Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Chapter 18 Opening Roadmap Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Introduction to Control of Gene Expression in Bacteria Cells does not express all genes all of the time Cells are very selective about: – Which genes they express – In what amounts genes are expressed – When genes are expressed Gene expression: – Gene product—RNA or a protein—is synthesized and becomes active – Controlled at every step Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved 18.1 An Overview of Gene Regulation and Information Flow Control over gene expression is crucial Thousands of bacterial species competing for space and nutrients along your intestinal wall: – Cells need to use resources efficiently – Synthesizing proteins not needed leaves fewer resources to make essential proteins – Bacterial gene expression triggered by specific signals from environment Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Mechanisms of Regulation (1 of 5) In bacteria—information flow occurs in three steps: DNA → mRNA → protein → activated protein Genes can be under: – Transcriptional control—mRNA only made for proteins needed – Translational control—Not all mRNAs are translated – Post-translational control—Proteins must be activated by chemical modification 3 Gene expression can be controlled at any of these steps Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Mechanisms of Regulation (2 of 5) Transcriptional control— most efficient yet slow: – Controls gene expression before cell expends many resources- saves energy Unneeded proteins are not transcribed into mRNA: – Therefore, ribosomes cannot make proteins Control occurs when regulatory proteins affect R NA polymerase’s ability to bind to promoter and initiate transcriptions: Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Mechanisms of Regulation (3 of 5) Translational control: – Allows more rapid changes because mRNA is already made Control can occur through many mechanisms: – Regulatory molecules can speed up m RNA degradation – Ability of mRNA to be translated can be affected Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Mechanisms of Regulation (4 of 5) Post-translational control: – Provides most rapid response – Activation by chemical modification can be controlled to regulate gene expression Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Figure 18.1 Gene Expression in Bacteria Can Be Regulated at Three Levels Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Bacteria need to be as energy efficient as possible Exhibit a strong preference for glucose as fuel Will switch to other fuels if needed but need new enzymes to metabolize the different fuel. Lactose is our example. This switching is controlled at the level of transcriptionmost energy efficient type of control, but with a cost in speed. Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Metabolizing Lactose―A Model System (1 of 2) E. coli uses lactose only when glucose is depleted Before E. coli can use lactose: – It must transport it into cell with protein galactoside permease – It must cleave it with enzyme β-galactosidase to produce glucose and galactose – These enzymes are not made unless the cell “needs” to use lactose. Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Figure 18.2 Two Proteins Needed to Use Lactose Are Galactoside Permease and 𝛃Galactosidase Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Metabolizing Lactose―A Model System (2 of 2) E. coli produces high levels of β-galactosidase only when lactose is present in the environment: – Lactose acts as an inducer – Molecule that triggers transcription of specific gene If glucose is also present, β-galactosidase is not produced in high levels: – Glucose negatively regulates β-galactosidase expression Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Logic chart for transcription of b-galactosidase GLUCOSE LACTOSE Present (Any) No transcription Absent Absent No transcription ? Absent Present Transcription ? b-galactosidase In the presence of glucose, b-galactosidase is not transcribed regardless of lactose levels If glucose is not available, then b-galactosidase will be transcribed if lactose is available Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Figure 18.4 Gene Transcription Is Regulated by Negative Control, Positive Control, or Both Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved 18.2 Negative and Positive Control of Transcription Transcription can be regulated with negative or positive control: – Negative control: ▪ Regulatory protein—repressor—binds to DN A and shuts down transcription – Positive control: ▪ Regulatory protein—activator—binds to DNA and triggers transcription The terms negative and positive refer to what happens when the protein in question is bound to DNA. Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved A Gene Needed to Regulate Lactose Metabolism (1 of 2) Monod and Jacob isolated and analyzed E. coli mutants that could not metabolize lactose: – To find genes that code for β-galactosidase or galactoside permease – Or to find genes that code for regulators of genes that code for these proteins They hoped to find regulators of lactose metabolism Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Identifying Regulated Genes Review: Isolating mutants with respect to a particular trait is a two-step process: 1. Generate a large number of individuals with mutations at random locations in their genomes (using X-rays, UV, chemical mutagens) 2. Screen the mutants to find individuals with defects in the process or pathway in question Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved A new kind of gene involved in Regulating Lactose Metabolism (2 of 2) Three classes of lactose metabolism mutants were identified: 1. lacZ− mutants could not cleave lactose because they lack functional β-galactosidase 2. lacY− mutants do not accumulate lactose in their cells because they lack galactoside permease 3. lacI− mutants produce β-galactosidase and galactoside permease even when lactose is absent: ▪ Called constitutive mutants ▪ Have a defect in gene regulation Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Table 18.1 Three Types of Lactose Metabolism Mutants Observed Phenotype Interpretation Genotype Cells cannot cleave lactose, even in the presence of inducer (lactose). No β-galactosidase; gene for βgalactosidase is defective. This gene is named lacZ. lacZ− Cells cannot accumulate lactose. No membrane protein (galactoside permease) to import lactose; gene for galactoside permease is defective. This gene is named lacY. lacY− Cells can cleave lactose even if lactose is absent as an inducer. Constitutive (constant) expression of lacZ and lacY; gene for regulatory protein that shuts down lacZ and lacY is defective. This gene is named lacI. lacI− Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Negative Control of Lactose Utilization Genes (1 of 2) lacI gene codes for a repressor: – Represses transcription of lacZ and lacY – Controls (regulates) expression of other genes Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Negative Control of Lactose Utilization Genes (2 of 2) The lacZ and lacY genes are under negative control: – The lacI gene codes for a repressor – Binds to DNA on or near the lacZ and lacY promoter lacI− mutants have no repressor: – lacZ and lacY genes are expressed with or without lactose In presence of lactose, the block by lacI is removed Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Figure 18.6 Genes Involved in Lactose Metabolism Are under Negative Control Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved The Operon Model (1 of 2) Jacob and Monod coined the term operon for: – Set of coordinately regulated bacterial genes that are transcribed together into one mRNA (polycistronic) Group of genes involved in lactose metabolism is termed lac operon A fourth gene, lacA, is also part of the lac operon (not discussed more) Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved The Operon Model (2 of 2) Three ideas are central to the operon model: 1. The lacZ, lacY, and lacA genes are adjacent and are transcribed into one mRNA initiated from single promoter of lac operon 2. ​lacI is a repressor of lac operon: ▪ Expressed constitutively ▪ Binds to the operator and blocks RNA polymerase 3. Lactose is the inducer that changes the shape of lacI, causing it to release from the operator: ▪ A type of allosteric regulation Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Figure 18.7 The lac Operon Is a Set of Coordinately Regulated Genes They are part of the same polycistronic transcript, all are transcribed when the promoter is active Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Positive Control of Lactose Utilization Genes (1 of 4) Transcription of the lac operon is greatly reduced when glucose is present: – Even when lactose is also available – When glucose is already available, cell does not need to produce more by cleaving lactose Two mechanisms for how glucose prevents lac operon expression: 1. Catabolite activator protein (CAP) 2. Control by inducer exclusion Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Positive Control of Lactose Utilization Genes (2 of 4) Positive control by CAP regulation: – CAP exerts positive control of lac operon – CAP is transcribed and translated constitutively – CAP binds to regulatory sequence upstream of promoter (CAP binding site): ▪ Increases frequency of transcription ▪ CAP must be bound to cyclic AMP (cAMP) in order to bind to D NA ▪ cAMP is related to low energy levels Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Positive Control of Lactose Utilization Genes (3 of 4) When glucose levels outside cell are low: – Cell is starving – CAP-cAMP complex stimulates RNA polymerase binding​to lac (and other operons) When glucose levels outside cell are high: – cAMP synthesis is inhibited- cell is happy – CAP-cAMP complex does not form – Transcription is not stimulated CAP binding site also found in other operons that function in the use of sugars other than glucose Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Positive Control of Lactose Utilization Genes (4 of 4) Control by Inducer Exclusion: – Second way for glucose to prevent lac operon expression – Glucose inhibits transport of sugars other than glucose into cell – Transport of lactose into cell is inhibited when glucose is high – When intracellular lactose is low, it does not bind to lacI-- does not remove repressor from operator – When glucose is low, more lactose enters and repressor is removed Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Figure 18.8 Two mechanisms by which Glucose Regulates the lac Operon Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Why Has the Lac Operon Model Been So Important Scientifically? The lac operon has been an immensely important model system: – Many bacterial genes and operons under negative control by repressor proteins – Activator proteins enhance binding of RNA polymerase to promoter – Gene transcription regulated by physical contact between regulator proteins and sequences in D N A Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Glossary of operon terminology Operator: a DNA sequence downstream of the promoter which is the binding site for a repressor Inducer: A small molecule which increases expression (of an operon) (example:lactose) Repressor: A protein which, when bound to DNA, reduces (operon) expression Activator: A protein which, when bound to DNA, increases (operon) expression Co-repressor: A small molecule which helps a repressor bind DNA Positive regulation: when a protein is bound, expression increases. Example: CAP/cAMP Negative regulation: when a protein is bound, expression decreases. Examples: lacI, tryptophan repressor Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Global Gene Regulation (1 of 2) Global gene regulation—coordinated regulation of many genes: – Needed for responses that require expression of dozens or even hundreds of genes Regulon—set of separate genes or operons: – Contains the same regulatory sequences – Controlled by single type of regulatory protein Regulons can be under negative control by a repressor or positive control by an activator protein Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Global Gene Regulation (2 of 2) SOS response regulon—under negative control: – Allows bacteria to repair DNA damage – DNA damage causes expression of over 40 genes – Code for enzymes needed for DNA repair, recombination, and specialized DNA polymerases SOS system is under control of its own genes: – LexA codes for LexA protein—represses transcription of SOS genes when cell is healthy – DNA damage sets off signal for LexA cleavage— leads to inactivation Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Figure 18.10 The Genes of the S O S Regulon Are Expressed Together Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved