Class Note for MIC 205A at UA 2
Class Note for MIC 205A at UA 2
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This 4 page Class Notes was uploaded by an elite notetaker on Friday February 6, 2015. The Class Notes belongs to a course at University of Arizona taught by a professor in Fall. Since its upload, it has received 13 views.
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Date Created: 02/06/15
Microbial Genetics The Operon Regulation of bacterial gene expression Controlling enzymes We know that Bacterial cells carry out a huge number of chemical reactions catalyzed by enzymes And Bacterial cells respond rapidlyto changing environments eg presence ofa new carbon source So control of Enzymes is VERY important and must be ef cient Enzyme Regulation Enzyme activity can be regulated by feedback inhibition of synthesized enzyme BUT making Enzyme requires Energy a a bacterium aririut make ALL Enzymes ALLTHE Tim so it is better in strip synthesizing Enzymes that are nut needed THUS many genes such as those for enzymes involved in lactose metabolism or amino acid production are regulated and only expressed when required naneven seem av genes are nm regulated and avelemied canenmive Regulation of gene expression Constitutive Enzymes No vEgulalian Enzyme induced Enzyme REPvessed Regulation of gene expression The process oftranscription plays an important role in the ability of bacteria to respond to changing environments mRNA compared to DNA is inherently unstable Thus the expression levels ofcertain proteins can be controlled at the level of transcription mRNA synthesis Transcriptional control I Two genetic control mechanisms which regulate transcription of mRNA and therefore the synthesis of corresponding protein I Inductionturning on gene expression I Repressionturning off gene expression Repression I Inhibition of gene expression New I Often in response to an excess of endproduct shuts down synthesis ofthe enzyme I Mediated by regulatory my proteins called repressors Black RNA pulymerase rrbrn initiating transeriptibn I Repressible genes are transcribed until they are NW repressed 3 quotWm a T em Induction I Turns on transcription of a gene I A substance which acts to induce transcription ofa gene is an inducer o en the substrate I Enzymes synthesized in the presence of inducers are called inducible enzymes I Example lactose utilization genes I Inducible genes are not transcribed until they are induced The operon ii ii igtigtigt 5 quotbut 3 I pman Dutor pmluns I Multiple genes are arranged in the same orientation and are closely linked on the DNA I Genes in an operon are transcribed on a single RNA transcript but are translated individually to form multiple proteins A rneenanisrnrbrebbrginate ebntrbi pr genes inybiyeg in a Single prbeess by regulating transeriptibn gr tne bperbn mRNA The operon cont I The term operon not only includes the structural genes in the operon but also the regulatory sequences controlling transcription I Promotersite of RNA polymerase binding I Operatorsite of binding of a regulatory protein Ci CI I gtIgtIgt Dmmer Dutor mRN A Operons and regulation I About 27 of E coli genes are in operons I Many ofthe genes arranged in operons are regulated I For example genes rbrtne iaetbse utilizatlun are ebntrbiieg by tne presenee gr iaetbse genes rbr pathugenesls are brten ebntrbiieg by ternperature ur pnysibibgie femur Two regulated operons I Two E coli operons re ect how operons in general are regulated I The lac operon I Encodes inducible catalytic enzymes involved in the lactose utilization and uptake I The arg operon I Encodes repressible anabolic enzymes involved in the production ofthe amino acid arginine The lac operon Three structural genes lacZ pegaiacmsmase splits lactuse tn glucuse and galactuse lacV a permease invulveu lrl transpurt ur lactuse lacA atlansaEEQlase functlun UHKHDWH A regulatory gehe ac The Lacl protein IS a repressorotthe acoper0ri A promoter Where RNA polymerase bil idS to transcribe the operon Ah operatorsite Where the Lacl repressor bil idS to block transcription Arrangement of the lac operon Operaquot eanmltegion Wgems r P o 2 v A Regulatory Promoter Operator gene In the absence of lactose I In the absence of lactose the repressor binds to operator site preventing binding of RNA polymerase and transcription In the presence of lactose The lac operon is I h an inducible operon wig Lactose acts as J the inducer Lactose induces enzyme yrquot 393 expression by binding to the LacI repressor preventing its 13 6 quotWWquot quot binding to the operator site 225 g RTFM The arg operon l A repressible operon genes are transcribed until turned off or repressed l Three structural genes argCBH encoding enzymes in the arginine biosynthetic pathway In the absence of arginine The arg operon is transcribed and enzymes for the mm rower synthesis of t i arginine are 3quot produced Alumwl miracuululv 7 J WWW o a u In the presence of arginine I Arginine acts as a corepressor and represses its own synthesis Like feedback inhibition but acts on enzyme synthesis rather than activity Summary of gene regulation l The production of many bacterial proteins are controlled by regulating transcription endproduct I Functionally related genes are arranged in operons to allow coordinate regulation I Operons can be induced inthe presence at substrate Repressed iri the presence at endpruduct I Transcripition mechanisms can be more complex with DNA sequences known as activators and attenuators
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