Ch18 Regulation of Gene expression 1 Chapter 18 Regulation of Gene Expression Overview Conducting the Genetic Orchestra o o o o o Both prokaryotes and eukaryotes alter their patterns of gene expression in response to changes in environmental conditions Multicellular eukaryotes also develop and maintain multiple cell types Each cell type contains the same genome but expresses a different subset of genes During development gene expression must be carefully regulated to ensure that the right genes are expressed only at the correct time and in the correct place Gene expression in eukaryotes and bacteria is often regulated at the transcription stage Control of other levels of gene expression is also important RNA molecules play many roles in regulating eukaryotic gene expressions Disruptions in gene regulation may lead to cancer Concept 18 1 Bacteria often respond to environmental change by regulating transcription Natural selection favors bacteria that express only those genes whose products are needed by the cell A bacterium in a tryptophan rich environment that stops producing tryptophan conserves its resources o Metabolic control occurs on two levels First cells can adjust the activity of enzymes already present This may happen by feedback inhibition in which the activity of the first enzyme in a pathway is inhibited by the pathway s end product o Feedback inhibition typical of anabolic biosynthetic pathways allows a cell to adapt to short term fluctuations in the supply of a needed substance Second cells can vary the number of specific enzyme molecules they make by regulating gene expression The control of enzyme production occurs at the level of transcription the synthesis of messenger RNA coding for these enzymes Genes of the bacterial genome may be switched on or off by changes in the metabolic status of the cell The basic mechanism for the control of gene expression in bacteria known as the operon model was described by Francois Jacob and Jacques Monod in 1961 o Operons the basic concept Escherichia coli synthesizes tryptophan from a precursor molecule in a series of steps with each reaction catalyzed by a specific enzyme The five genes coding for the subunits of these enzymes are clustered together on the bacterial chromosome as a transcription unit served by a single promoter Transcription gives rise to one long mRNA molecule that codes for all five polypeptides in the tryptophan The mRNA is punctuated with start and stop codons that signal where the coding sequence for each polypeptide A key advantage of grouping genes with related functions into one transcription unit is that a single on off switch can control a cluster of functionally related genes In other words these genes are coordinately controlled When an E coli cell must make tryptophan for itself all the enzymes are synthesized at one time The switch is a segment of DNA called an operator The operator located within the promoter or between the promoter and the enzyme coding genes controls the access of RNA polymerase to the genes pathway begins and ends o o The operator the promoter and the genes they control constitute an operon The trp operon trp for tryptophan is one of many operons in the E coli genome Ch18 Regulation of Gene expression 2 By itself an operon is turned on RNA polymerase can bind to the promoter and transcribe the genes of the operon o The operon can be switched off by a protein called the trp repressor The repressor binds to the operator blocks attachment of RNA polymerase to the promoter and prevents transcription of the operon s genes Each repressor protein recognizes and binds only to the operator of a particular operon The trp repressor is the protein product of a regulatory gene called trpR which is located at some distance from the operon it controls and has its own promoter Regulatory genes are transcribed continuously at slow rates and a few trp repressor molecules are always present in an E coli cell Why is the trp operon not switched off permanently First binding by the repressor to the operator is reversible An operator vacillates between two states with and without a repressor bound to it The relative duration of each state depends on the number of active repressor molecules Second repressors contain allosteric sites that change shape depending on the binding of other o o o o o o around molecules The trp repressor has two shapes active and inactive The trp repressor is synthesized in an inactive form with little affinity for the trp operator Only if tryptophan binds to the trp repressor at an allosteric site does the repressor protein change to the active form that can attach to the operator turning the operon Tryptophan functions in the trp operon as a corepressor a small molecule that cooperates with a repressor off protein to switch an operon off When concentrations of tryptophan in the cell are high more tryptophan molecules bind with trp repressor molecules activating them The active repressors bind to the trp operator and turn the operon off At low levels of tryptophan most of the repressors are inactive and transcription of the operon s genes resumes o Repressible and Inducible Operons Two types of negative gene regulaion The trp operon is an example of a repressible operon one that is inhibited when a specific small molecule tryptophan binds allosterically to a regulatory protein In contrast an inducible operon is stimulated induced when a specific small molecule interacts with a regulatory protein The classic example of an inducible operon is the lac operon lac for lactose Lactose milk sugar is available to E coli in the human colon if the host drinks milk Lactose metabolism begins with hydrolysis of lactose into its component monosaccharides glucose and galactose o This reaction is catalyzed by the enzyme galactosidase Only a few molecules of galactosidase are present in an E coli cell grown in the absence of Ch18 Regulation of Gene expression 3 o o lactose If lactose is added to the bacterium s environment the number of galactosidase molecules increases by a thousandfold within 15 minutes The gene for galactosidase is part of the lac operon which includes two other genes coding for enzymes that function in lactose metabolism The regulatory gene lacI located outside the operon codes for an allosteric repressor protein that can switch off the lac operon by binding to the operator Unlike the trp operon the lac repressor is active all by itself binding to the operator and