L17 Gene regulation in bacteria

L17 Gene regulation in bacteria - Gene regulation in...

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Unformatted text preview: Gene regulation in bacteria 1 Last time ✤ Overview of transcription ✤ Signposts - recognizing patterns of sequence Three stages: Initiation, Elongation, Termination ✤ ✤ Specific mechanisms of bacterial transcription 2 Gene regulation in bacteria ✤ Gene regulation overview Example of transcriptional control of gene expression: The lac operon ✤ ✤ How (and when) the lac operon is repressed How (and when) the lac operon is induced in transcriptional gene regulation ✤ ✤ Themes 3 Gene regulation The level of expression of most genes varies under different conditions Some genes are ‘on’ all the time (constitutive genes) http://bhs.smuhsd.org/bhsnew/academicprog http://www.lebs.cnrs-gif.fr/golinelli/images/trna4.jpg http://www.srh.noaa.gov/lch/prep/heat.jpg 4 Why turn genes off or on? pro.corbis.com/images/42-19215668.jpg It saves the cell some energy! 5 There are lots of ways to regulate gene expression ✤Gene organization (operon promotes co-expression of related genes) ✤Transcription (repression, activation, attenuation) ✤mRNA stability (affected by translation and the 3’ stemloops, have “degradosome”) ✤Translation (repression, activation and autoregulation) ✤Protein stability and other modifications 6 There are lots of ways to regulate gene expression ✤Gene organization (operon promotes co-expression of related genes) ✤Transcription (repression, activation, attenuation) ✤mRNA stability (affected by translation and the 3’ stemloops, have “degradosome”) ✤Translation (repression, activation and autoregulation) ✤Protein stability and other modifications 7 Transcriptional initiation Genes are not so much turned “on” and “off” as they are expressed “more” or “less” more like this not like this pro.corbis.com/images/42-19215668.jpg http://images.lowes.com/general/d/dimmer_switch_plate.jpg 8 Regulatory proteins Promoter Terminator Regulatory sequence bound by proteins: Activators - proteins that Repressors - proteins bind DNA to stimulate that bind DNA to or inhibit transcription transcription (positive control) (negative control) 9 Effector molecules: Inducers no inducer with inducer inducible gene repressor protein inducer inducible gene activator protein inducer 10 Effector molecules: co-repressor no co-repressor with co-repressor repressor protein co-repressor 11 Effector proteins: Inhibitors no inhibitor with inhibitor repressible gene inhibitor activator protein 12 One more thing…. Organization of genes in bacteria: the operon promoter terminator CAP site operator structural genes operon: a group of co-ordinately regulated genes 13 Jacob and Monod’s observation E.coli exposed to lactose - lac operon protein levels rise Remove lactose - lac operon protein levels fall lactose 14 Diauxic growth of E. coli on a mixture of lactose + glucose. "bi-phasic" growth curve Francois Jacques Jacob Monod 15 Lactose metabolism Permease 16 Lac Operon and associated genes LacI repressor Operator: Binding site for repressor (lacI) Permease β galactosidase rearranges lactose to form allolactose, the inducer, also breaks down lactose into glc and gal galactoside transacetylase http://en.wikipedia.org/wiki/Lac_operon 17 no lactose, repressor on lactose, repressor off lac genes expressed 18 lac operon expression is controlled Mechanism of repression by LacI (a repressor protein) - relieved by allolactose, an effector molecule (inducer) Mechanism of activation by CAP (an activator protein) - induced by cAMP, an effector molecule (inducer) 19 Mechanism of repression by LacI (weak promoter) 20 Allolactose relieves repression operon is induced 21 Wait a minute...huh? gene expression is more or less, not on or off the lac promoter is ‘leaky’ 22 Glucose (indirectly) controls lac operon expression 23 Introducing another effector molecule: cAMP glucose adenylyl cyclase ATP cAMP + PPi 24 cAMP binds activator (CAP) to induce lac operon expression + = - Stimulates the lac operon - cAMP binds to CAP (Activator protein) - cAMP-CAP has high affinity for DNA 25 When lactose is present, and glucose is absent CAP cAMP 26 Hypothesis for CAP-cAMP dimer interacting with the α subunit of the RNAP Core RNAP bound more tightly - means better transcription! 27 No lactose or glucose present 28 Lactose and glucose present 29 Glucose present, but no lactose 30 There are actually THREE operators in lac operon http://us.f308.mail.yahoo.com/ym/login?.rand=72n418e1t55hu 31 Mutational analysis of operators 32 LacI binds O1 and either O2 or O3 Promoter sequence 33 Themes 34 Summary ✤ Gene regulation overview Example of transcriptional control of gene expression: The lac operon ✤ ✤ How (and when) the lac operon is repressed How (and when) the lac operon is induced in transcriptional gene regulation ✤ ✤ Themes 35 Ungraded problems ✤ Look over Chapter 14 S1, S5 ✤ Try Chapter 14 C1 - C3, C5, C9 ✤ Try Chapter 14 E5 36 Next Time Transcription in Eukaryotes http://www.mun.ca/biochem/courses/3107/images/yeast_RNAPII.gif 37 ...
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This note was uploaded on 03/29/2009 for the course BIO 325 taught by Professor Saxena during the Fall '08 term at University of Texas at Austin.

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