8 Регуляция экспрессии генов. Система передачи сигнала (1160077), страница 8
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This system responds to temperature increases as well as someother environmental stresses, and it involves theinduction of a set of proteins. Binding of RNA polymerase to heat-shock promoters is mediated by aspecialized a subunit of the enzyme called cr32,which replaces <x70.LRNA start siteDNA 5TNTCNCCCTTGAAN _13 15CCCCATTTA N7mRNARepressors bind to specific sites in the DNA. In prokaryotes, thebinding sites for repressors are called operators.
Operator sites aregenerally near and often overlap the promoter so that RNA polymerasebinding, or its movement along the DNA after binding, is blockedwhenever the repressor is present. Regulation by means of a repressorNegative regulationPositive regulation(bound repressor inhibits transcription)(bound activator facilitates transcription)(c)(a)RNA polymeraseOperatorMolecular signal( ^ ) causes dissociationof regulatory proteinfrom DNADNAPromotermRNAmRNA(d)(b)Molecular signal( ^ ) causes bindingof regulatory proteinto DNAmRNAmRNAprotein that binds to DNA and blocks transcription is referred to asnegative regulation. Repressor binding is regulated by a molecularsignal, usually a specific small molecule that binds to and induces aconformational change in the repressor.
The interaction between repressor and signal molecule may lead to either an increase or a decrease in transcription. In some cases the conformational change results in dissociation of a DNA-bound repressor from the operator (Fig.27-4a). Transcription initiation can then proceed unhindered. In othercases the interaction between an inactive repressor and the signal molecule causes the repressor to bind to the operator (Fig.
27-4b).Activators provide a molecular counterpoint to repressors. Regulation mediated by an activator is called positive regulation. Activators bind to sites adjacent to a promoter and enhance the binding andactivity of RNA polymerase at that promoter. The binding sites foractivators are often found adjacent to promoters that are normallybound weakly or not at all by RNA polymerase.
Transcription at thesegenes is therefore often negligible in the absence of activator. Sometimes the activator is normally bound to DNA and dissociates when itbinds to the signal molecule, often a specific small molecule or anotherprotein (Fig. 27-4c). When bound to the DNA, the activator proteinfacilitates RNA polymerase binding and increases the rate of transcription initiation. In other cases the activator is not bound to theDNA until it also binds to a molecular signal (Fig.
27-4d). Positiveregulation is particularly common in eukaryotes, as we shall see. Wenow turn to a fundamental unit of gene expression, the study of whichgave rise to much of our current understanding of the regulation ofgene expression.Figure 27-4 Common patterns of regulation oftranscription initiation. Two types of negative regulation are illustrated, (a) The repressor (red) isbound to the operator in the absence of the molecular signal; the signal causes dissociation of the repressor to permit transcription, (b) The repressor isbound in the presence of the signal; the repressordissociates and transcription ensues when the signal is removed.
Positive regulation is mediated bygene activators, (c) The activator (green) binds inthe absence of the molecular signal and transcription proceeds; the activator dissociates and transcription is inhibited when the signal is added,(d) The activator binds in the presence of the signal; it dissociates only when the signal is removed.Note that "positive" and "negative" regulation aredefined by the type of regulatory protein involved.In either case the addition of the molecular signalmay increase or decrease transcription, dependingon the effect of the signal on the regulatory protein.Part IV Information Pathways946Figure 27—5 An operon.
Genes A, B, and C aretranscribed on one polycistronic mRNA. Typicalregulatory sequences include binding sites for proteins that either activate or repress transcriptionfrom the promoter.Repressorbinding site(operator)Activatorbinding siteIDNA\| PromoterWllllllillll/\ Regulatory sequences1cAJVVGenes transcribed as a unitMany Prokaryotic Genes Are Regulatedin Units Called OperonsLactoseGalactoside permeaseOutsideHHOHOHLactose/3-galacto.sidaseO-CH 2?/4~°\?HHOHGalactoseHOHGlucoseBacteria have a simple general mechanism for coordinating the regulation of genes whose products are involved in related processes: thegenes are clustered on the chromosome and transcribed together.
Mostprokaryotic mRNAs are polycistronic. The single promoter required toinitiate transcription of the cluster is the point where expression of allof the genes is regulated. The gene cluster, the promoter, and additional sequences that function in regulation are together called an operon (Fig. 27-5). Operons that include 2 to 6 genes transcribed as aunit are common; some operons contain 20 or more genes.Many of the principles guiding the regulation of gene expression inbacteria were defined by studies of the regulation of lactose metabolism in E. coli.
The disaccharide lactose can be used as the sole carbonsource for the growth of E. coli. In 1960, Frangois Jacob and JacquesMonod published a short paper in the Proceedings of the French Academy of Sciences demonstrating that two genes involved in lactose metabolism were coordinately regulated by a genetic element located adjacent to them. The genes were those for /3-galactosidase, which cleaveslactose to galactose and glucose, and galactoside permease, whichtransports lactose into the cell (Fig.
27-6). The terms operon and operator were first introduced in this paper. The operon model that evolvedfrom this and subsequent studies permitted biochemists to think aboutgene regulation in molecular terms for the first time.The lac Operon Is Subject to Negative RegulationThe model for regulation of the lactose (lac) operon deduced from thesestudies is shown in Figure 27-7; it follows the pattern outlined inFigure 27-4a. In addition to the genes for /3-galactosidase (Z) and galactoside permease (Y), the operon includes a gene for thiogalactosidetransacetylase (A), whose physiological function is unknown. Each ofthe three genes is preceded by translational signals (not shown in Fig.27-7) to guide ribosome binding and protein synthesis (Chapter 26). InFigure 27-6 The activities of galactoside permeaseand /3-galactosidase in lactose metabolism inE.
coli. The conversion of lactose to allolactose bytransglycosylation is a minor reaction catalyzed by/3-galactosidase.947Chapter 27 Regulation of Gene ExpressionFigure 27-7 The lac operon in the repressedstate. The I gene encodes the Lac repressor. Thelac Z, Y, and A genes encode /3-galactosidase, galactoside permease, and transacetylase, respectively.The P and 0 sites are the promoter and operatorfor the lac genes, respectively. The Pi site is thepromoter for the I gene.?Repressor ^{5^§c f~mRNA\/\/\/DNAPxthe absence of the substrate lactose, the lac operon genes are repressed, and j3-galactosidase is present in only a few copies (a few molecules) per cell. Jacob and Monod found that mutations in the operatoror in another gene called I led to constitutive synthesis of the lac operon gene products.
When the I gene was defective, repression could berestored by introducing a functional I gene to the cell on another DNAmolecule. This showed that the I gene encoded a diffusible moleculethat caused gene repression; the molecule was later shown to be aprotein, now called the Lac repressor. Repression is not absolute. Evenin the repressed state each cell has a few copies of /3-galactosidase andgalactoside permease, presumably synthesized on the rare occasionswhen the repressor briefly dissociates from its DNA binding site (theoperator).When cells are provided with lactose, the lac operon is induced.
Aninducer molecule binds to a specific site on the repressor causing aconformational change in the repressor that results in its dissociationfrom the operator (Fig. 27-8). The inducer in this system is not lactoseitself but an isomer of lactose called allolactose (Fig. 27-6). Lactoseentering the E. coli cell is converted to allolactose in a reaction catalyzed by the few copies of /3-galactosidase in the cell. Allolactose thenbinds to the Lac repressor. After the repressor dissociates, the lac operon genes are expressed and the concentration of /3-galactosidase increases by a factor of 1,000.Jacques MonodFrangois JacobDNAAllolactose(or IPTG)YmRNAAFigure 27-8 Induction of the lac operon in response to a molecular signal.
Binding of allolactoseto the Lac repressor causes a conformationalchange. The repressor dissociates from the operator, allowing transcription to proceed. Other/3-galactosides, such as isopropylthiogalactoside(IPTG), can also act as inducers.Part IV Information Pathways948Several /3-galactosides structurally related to allolactose are inducers of, but not substrates for, /3-galactosidase, and some are substratesbut not inducers. One particularly effective and nonmetabolizable inducer of the lac operon often used experimentally is isopropylthiogalactoside (IPTG). Such nonmetabolized inducers permit the separation of the physiological function of lactose as a carbon source forgrowth from its function in the regulation of gene expression.Many operons are now known in bacteria and a few have beenfound in lower eukaryotes.
The mechanisms by which they are regulated can vary significantly from the simple model presented in Figure27-7. Research has shown that even the lac operon is more complexthan indicated here, with an activator protein also contributing to theoverall scheme. The regulation of several well-studied bacterial operons, including lac, is described in more detail later in this chapter. Wenow consider the critical molecular interactions between DNA-bindingproteins (e.g., repressors and activators) and the specific DNA sequences to which they bind.CH2OHHOHIsopropylthiogalactoside(IPTG)Regulatory Proteins Have Discrete DNA-Binding DomainsFigure 27—9 Functional groups on DNA base pairsin the major groove of DNA. The groups that canbe used for base-pair recognition are shown in redfor all four base pairs.Major grooveRegulatory proteins generally bind to specific DNA sequences. Theyalso bind to nonspecific DNA, but their affinity for their target sequences is generally 105 to 107 times higher.
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