Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Chapter 12: Gene expression Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 12-1 Regulation of gene expression • Some proteins are required continuously or often—the genes encoding them are expressed constitutively • Many other proteins are required for specific purposes—the genes encoding them are differentially regulated Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 12-2 Control points for expression • Transcriptional regulation • Post-transcriptional regulation – includes co-transcriptional control—RNA splicing – mRNA stability and transport • Translation • Post-translational regulation – protein stability – activation or inactivation, e.g. cleavage or phosphorylation Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 12-3 Regulation of gene expression in prokaryotes • Most gene regulation is transcriptional in prokaryotes • Prokaryotic genes are located in functional groups • Genes encoding enzymes in the same pathway are arranged in sequence under the control of a single promoter Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 12-4 Regulation of gene expression in prokaryotes (cont.) • The genes and their regulatory sequences are called an operon • Transcription from the promoter produces multigene RNA transcripts • Multiple polypeptides are translated from these RNAs Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 12-5 Induction • Expression of inducible genes is either positive or negative at the level of transcription • Positive regulation (induction) is often used in the catabolism of compounds for energy production— the presence of the molecule stimulates expression of genes in the pathway Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 12-6 Repression • Negative regulation (repression) may be used in anabolic or synthetic pathways, where the presence of the synthesised compound represses the expression of the genes in the pathway • Thus the genes are only expressed in the absence of the compound Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 12-7 Fig 12.12: A bacterial operon The lac operon (in Escherichia coli), responsible for the metabolism of lactose to glucose and galactose Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 12-8 Regulation of the lactose (lac) operon • The genes in the lac operon are induced in the presence of lactose • The lac operon is an example of negative regulation—the genes are repressed in the absence of lactose • The repressor protein binds to lacO, the operator region of the lac promoter, and prevents transcription by blocking RNA polymerase binding to the promoter Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 12-9 Regulation of the lac operon (cont.) • A metabolite of lactose called allolactose binds to a repressor protein and changes its shape, preventing binding to the lac promoter • This is called an allosteric interaction and is a common molecular regulatory mechanism • As lactose is used up in a cell, the free repressor protein can bind to the promoter and prevent transcription Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 12-10 Fig 12.4a: Regulation of the lac operon Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 12-11 Fig 12.4b and c: Regulation of the lac operon (cont.) (b) (c) Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 12-12 Regulation of the lac operon (cont.) • The lac operon is also positively regulated • When glucose is present as well as lactose, the lac operon is repressed to allow metabolism of glucose • This is mediated by the catabolite activator protein (CAP) • cAMP normally binds to CAP protein (Fig. 12.5) allowing transcription from the operon in the absence of repressor binding Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 12-13 Regulation of the lac operon (cont.) • High levels of glucose lower cAMP concentration • The CAP–cAMP complex cannot form and transcription is reduced even in the presence of lactose • The double regulation of the lac operon ensures that it is only expressed at low glucose concentrations when lactose is present Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 12-14 Fig 12.5: Regulation of the lac operon in (a) low glucose and (b) high glucose conditions (a) (b) Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 12-15 Fig 12.6: Expression of the lac operon Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 12-16 Question 1: Predict what the outcome of a mutant of the repressor gene would be a) The lac genes would be expressed efficiently only in the absence of lactose b) The lac genes would be expressed efficiently only in the presence of lactose c)The lac genes would be expressed continuously d) The lac genes would never be expressed efficiently Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 12-17 Regulation of the tryptophan biosynthesis (trp) operon • When tryptophan is present in an environment, it can be more efficient to import than to synthesise • The trp operon contains five genes for tryptophan biosynthesis under the control of a single promoter • The absence of tryptophan leads to trp operon expression • Regulatory events are similar to lac operon • The trp repressor protein binds to the operator sequence only in the presence of tryptophan Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 12-18 Regulation of the tryptophan biosynthesis (trp) operon (cont.) • Tryptophan binds to the trp repressor protein, changing its conformation and allowing binding • Transcription cannot proceed • Low levels of tryptophan prevent repressor binding • Transcription and synthesis of tryptophan proceed Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 12-19 Fig 12.7: Regulation of the trp operon Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 12-20 Regulation of gene expression in eukaryotes • Most eukaryotic genes are controlled at the level of transcription • The mechanisms involve trans-acting regulatory proteins interacting with cis-acting control sequences on the DNA • DNA control sequences include promoters, promoter-proximal elements, enhancers and silencers Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 12-21 Regulation of gene expression in eukaryotes (cont.) • Eukaryotic promoters do not provide sufficient recognition signals for RNA polymerase II to initiate transcription in vivo • Core promoter elements must each be recognised by regulatory proteins (basal factors and coactivators) which bind to these sites • RNA polymerase II is bound and initiated properly and transcription commences Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 12-22 Fig 12.8: Regulation of transcription in eukaryotic cells Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 12-23 Yeast GAL regulation • The yeast Saccharomyces cerevisiae is a good model of eukaryote gene expression • In the presence of a medium containing galactose, five genes are expressed • Unlike the genes in an operon, these five genes are separate and transcribed independently • The expression of the genes is controlled by both positive and negative protein regulators Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 12-24 Yeast GAL regulation (cont.) • The GAL4 (Gal4p) protein binds to an upstream DNA activator sequence (UAS) on all five genes and induces their expression • The GAL80 protein (Gal80p) acts as a transcription repressor by binding to Gal4p, preventing the induction of transcription • When galactose is present, a metabolite dissociates the Gal80p from the Gal4p, allowing transcription to proceed Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 12-25 Fig 12.11: Regulation of galactose (GAL)-inducible genes in yeast Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 12-26 Combinations of enhancers and silencers • Series of DNA sequences that are recognised and bound to by DNA-binding proteins called transcription factors • Enhancers and silencers can, however, act over longer distances than promoters, and may be positioned either upstream or downstream of the promoter they enhance • Each enhancer or silencer element acts independently of others Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 12-27 Gene regulatory cascades in eukaryotes • Transcription factors bind to enhancer or silencer sequences • They are often tissue-specific, allowing a complex pattern of gene expression • Transcription factors are themselves gene products • Coordinated expression of many genes can therefore be achieved by just a few genes • Such regulatory cascades are important in embryonic development of eukaryotes Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 12-28 RNA-mediated regulation of gene expression • Not all regulatory genes act by producing a protein product • Small RNA molecules have been discovered that bind directly to mRNA and block their translation • These ‘microRNAs’ provide another level of gene regulation Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 12-29 Fig 12.12: Control of gene expression by microRNAs Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 12-30 Question 2: A repressible operon (e.g. the trp operon) is turned ‘off’ when: a) The repressor binds to the gene b) The repressor–corepressor complex binds to the operator c) The repressor binds to DNA polymerase d) The repressor does not bind to the gene Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 12-31 Summary • Gene expression is controlled • Genes may be expressed constantly or differentially regulated • Prokaryotic genes are regulated primarily at the level of transcription • Eukaryotic transcription requires activation by transcription factors • Non-protein products can regulate gene expression – microRNAs Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 12-32