8 Регуляция экспрессии генов. Система передачи сигнала (1160077), страница 11
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Mostof these proteins have distinct DNA-binding domains. Within these domains, common structuralmotifs involved in DNA binding are the helix-turnhelix and zinc finger motifs. Regulatory proteinsalso contain domains for protein-protein interactions, including leucine zipper and helix-loop-helixmotifs involved in dimerization and several classesof domains involved in the activation of transcription.The lactose operon of E. coli also exhibits positive regulation by the catabolite gene activator protein (CAP). When cAMP concentrations are high(glucose concentrations are low), CAP binds to aspecific site on the DNA, stimulating transcriptionof the lac operon and production of lactose-metabolizing enzymes. The presence of glucose depressescAMP concentrations, restricting expression of lac(and other) genes and suppressing the use of secondary sugars.
Several operons that are coordinately regulated, as with CAP and cAMP, are referred to as a regulon.Other mechanisms of regulation are also observed in prokaryotes. In the arabinose (ara) operon, the AraC protein acts as both activator andrepressor. Some repressors, as in the ara operonand the bacteriophage A system, regulate theirown synthesis (autoregulation). Some regulatoryproteins in the ara system bind sites many basepairs distant from each other and interact by DNAlooping mechanisms.
Amino acid biosynthetic operons have a regulatory circuit called attenuationthat uses a transcription termination site (the attenuator), modulating its formation in the mRNAby a mechanism that couples transcription andtranslation and responds to small changes inamino acid concentration. In the SOS system, multiple unlinked genes are repressed by a single typeof repressor protein, and all of the genes are induced simultaneously when DNA damage triggersRecA protein-mediated proteolysis of the repressor. The bacteriophage A has a complex regulatorycircuit that oversees the choice between lysis andlysogeny.
Two A proteins, N and Q, act as antiterminators, modifying the host RNA polymerase sothat it can bypass transcription termination sites.Finally, some prokaryotic genes are regulated bygenetic recombination processes that physicallymove promoters relative to the genes being regulated. These diverse mechanisms permit very sensitive cellular responses to changes in environmental conditions.Some regulation also occurs at the level oftranslation. The synthesis of ribosomal proteins inbacteria is mediated by a strategy in which oneprotein in each ribosomal protein operon acts as a982Part IV Information Pathwaystranslational repressor.
The mRNA is bound by therepressor and translation is blocked only when theribosomal protein is present in excess relative toavailable rRNA.Eukaryotes employ many of the same regulatory schemes, although positive regulation appearsto be more common and transcription is also accompanied by large changes in chromatin structure. Eukaryotic transcriptional activator proteinsare generally required for RNA polymerase binding and activity. Some transcription factors havegeneral functions; the TFII factors associated withRNA polymerase II, for example, are required atalmost all RNA polymerase II promoters.
Othertranscriptional activators, unique to one gene orset of genes, have distinct domains for DNA binding and activation, and their DNA binding sites areoften found hundreds of base pairs from the sitewhere RNA synthesis begins.Perhaps the most complex regulatory problemis the development of a multicellular animal. Here,sets of regulating genes operate in temporal andspatial succession, turning a given area of an eggcell into a predictable structure in the adult animal. Research continues into the molecular basisfor this highly coordinated process.Further ReadingGeneralJacob, F.
& Monod, J. (1961) Genetic regulatorymechanisms in the synthesis of proteins. J. Mol.Ingraham, J.L., Magasanik, B., Low, K.B.,Biol. 3, 318-356.Schaechter, M., & Umbarger, H.E. (eds) (1987)The operon model and the concept of messengerEscherichia coli and Salmonella typhimurium,RNAwere proposed in this historic paper.Cellular and Molecular Biology, Vol.
2, AmericanSociety for Microbiology, Washington, DC.Nomura, M., Gourse, R., & Baughman, G. (1984)An excellent reference source for reviews of manyRegulation of the synthesis of ribosomes and ribobacterial operons.somal components. Annu. Rev. Biochem. 53, 75117.Pabo, CO. & Sauer, R.T. (1992) Transcription factors: structural factors and principles of DNA recPtashne, M., Johnson, A.D., & Pabo, CO. (1982)ognition. Annu.
Rev. Biochem. 61, 1053-1095.A genetic switch in a bacterial virus. Sci. Am. 247(November), 128-140.Schleif, R. (1986) Genetics and Molecular Biology,Addison-Wesley Publishing Co., Inc., Reading, MA.Stephens, J.C., Artz, S.W., & Ames, B.N. (1975)Chapters 12, 13, and 14 provide an excellent acGuanosine5'-diphosphate3'-diphosphatecount of the experimental basis of major concepts of(ppGpp): positive effector for histidine operon trangene regulation in prokaryotes.scription and general signal for amino acid deficiency. Proc. Natl.
Acad. Sci. USA 72, 4389-4393.Schleif, R. (1992) DNA looping. Annu. Rev. Biochem. 61, 199-223.Yanofsky, C. (1981) Attenuation in the control ofexpression of bacterial operons. Nature 289, 751Struhl, K. (1989) Helix-turn-helix, zinc-finger, and758.leucine-zipper motifs for eukaryotic transcriptional regulatory proteins. Trends Biochem.
Sci.Zieg, J., Silverman, M., Hilmen, M., & Simon, M.14, 137-140.(1977) Recombinational switch for gene expression.Science 196, 170-172.Watson, J.D., Hopkins, N.H., Roberts, J.W.,Steitz, J.A., & Weiner, A.M. (1987) Molecular BiRegulation of Gene Expression in Eukaryotesology of the Gene, 4th edn, The Benjamin/Beardsley, T. (1991) Smart genes.
Sci. Am. 265Cummings Publishing Company, Menlo Park, CA.(August), 86-95.A good overview of gene regulation during developRegulation of Gene Expression inment.ProkaryotesGottesman, S. (1984) Bacterial regulation: globalregulatory networks. Annu. Rev. Genet. 18, 415441.DeRobertis, E.M., Oliver, G., & Wright, C.V.E.(1990) Homeobox genes and the vertebrate bodyplan. Sci. Am. 263 (July), 46-52.Chapter 27 Regulation of Gene Expression983Guarente, L. (1988) UASs and enhancers: commonmechanism of transcriptional activation in yeastand mammals. Cell 52, 303-305.Pugh, B.F. & Tjian, R.
(1992) Diverse transcriptional functions of the multisubunit eukaryoticTFIID complex. J. Biol. Chem. 267, 679-682.Kornberg, R.D. & Lorch, Y. (1991) Irresistibleforce meets immovable object: transcription andthe nucleosome. Cell 67, 833-836.Struhl, K. (1987) Promoters, activator proteins,and the mechanism of transcriptional initiation inyeast. Cell 49, 295-297.McKnight, S.L. (1991) Molecular zippers in generegulation.
Sci. Am. 264 (April), 54-64.A good description of leucine zippers.Thummel, C.S. (1992) Mechanisms of transcriptional timing in Drosophila. Science 255, 39-40.Melton, D.A. (1991) Pattern formation during animal development. Science 252, 234-241.Zlatanova, J. (1990) Histone HI and the regulationof transcription of eukaryotic genes. Trends Biochem. Sci. 15, 273-276.Ptashne, M. (1989) How gene activators work.
Sci.Am. 260 (January), 40-47.Problems1. Negative Regulation In the lac operon, describethe probable effect on gene expression of:(a) Mutations in the lac operator(b) Mutations in the lad gene(c) Mutations in the promoter2. Effect ofmRNA and Protein Stability on Regulation An E.
coli cell is growing in a solution withglucose as the sole carbon source. Tryptophan issuddenly added. The cells continue to grow, anddivide every 30 min. Describe (qualitatively) howthe amount of tryptophan synthase activity in thecell changes if:(a) The trp mRNA is stable (degraded slowlyover many hours).(b) The trp mRNA is degraded rapidly, but tryptophan synthase is stable.(c) The trp mRNA and tryptophan synthase areboth degraded rapidly.3. Functional Domains in Regulatory Proteins Abiochemist replaces the DNA-binding domain ofthe yeast GAL4 protein with the DNA-bindingdomain from the A repressor (CI) and finds that theengineered protein no longer functions as a transcriptional activator (it no longer regulates transcription of the gal operon in yeast).
What might bedone to the GAL4 DNA-binding site to make theengineered protein functional in activating galoperon transcription?4. Bacteriophage A Bacteria that become lysogenic for bacteriophage A are immune to subsequent A lytic infections. Why?5. Regulation by Means of Recombination In thephase variation system of Salmonella, what wouldhappen to the cell if the Hin recombinase becamemore active and promoted recombination (theswitch) several times in each cell generation?6. Transcription Attenuation In the leader regionof the trp mRNA, what would be the effect of:(a) Increasing the distance (number of bases)between the leader peptide gene and sequence 2?(b) Increasing the distance between sequences 2and 3?(c) Removing sequence 4?7.
Specific DNA Binding by Regulatory Proteins Atypical prokaryotic repressor protein discriminatesbetween its specific DNA-binding site (operator)and nonspecific DNA by a factor of 105 to 106. Aboutten molecules of the repressor per cell are sufficient to ensure a high level of repression. Assumethat a very similar repressor existed in a humancell and had a similar specificity for its bindingsite. How many copies of the repressor would berequired per cell to elicit a level of repression similar to that seen in the prokaryotic cell? (Hint: TheE.
coli genome contains about 4.7 million basepairs and the human genome contains about 2.4billion base pairs.)8. Positive Regulation A new RNA polymeraseactivity is discovered in crude extracts of cells derived from an exotic fungus. The RNA polymeraseinitiates transcription only from a single, highlyspecialized promoter. As the polymerase is purified, its activity is observed to decline. The purifiedenzyme is completely inactive unless crude extractis added to the reaction mixture. Suggest an explanation for these observations..
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