Explanation ofWheninducerisadded,synthesisexperimental results:ofβ-galactosidasecontinues.With no inducer,Initial lackofrepressorβ-galactosidasesynthesisstops;allowed synthesis ofLacZlacitis beingexpressed,enzyme. As LacIisproducingalacrrepressor.produced, LacZ synthesisgenes introducedwas shut off, unless anSynthesisofinduceris present toβ-galactosidasebeginsshortlyafterlacand lacztprevent repressor fromgeneswereintroduced;lacztgeneisbeingexpressed1repressing123456708Time (hours)Conclusion:lacl encodes the negative regulator of the lac genes17-16
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display 17-16 Explanation of Explanation of experimental results: experimental results: Initial lack of repressor Initial lack of repressor allowed synthesis of allowed synthesis of LacZ enzyme. As enzyme. As LacI is produced, produced, LacZ synthesis synthesis was shut off, unless an was shut off, unless an inducer is present to inducer is present to prevent repressor from prevent repressor from repressing. repressing. Conclusion: lacI encodes the negative regulator of the lac genes
Mutational and structural analysis of the repressor(a)RepressionPromoterOperatorPlacyOlaczlacADNAlac/+laclnormalmutantrepressorrepressonCanbindCannot bindto operator.to operator.EnzymesareEnzymesproducednotproducedconstitutively.(b)Inductionlac/slac/+superrepressormutantInducerInducercanbindtotnducer cannotbindrepressorandaltertorepressor.Repressortheproteinsoitcannotbindstooperon,evenbindto operator.wheninducerispresent.EnzymesareproducedEnzymesarenotproduced17-17Fig. 15.7, 15.8
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display 17-17 Mutational and structural analysis of the repressor Mutational and structural analysis of the repressor Fig. 15.7, 15.8
Changes in the operator DNA to which the repressorbinds can also affect repressor activityMutant operator (oc)PlaczlacYlacANucleotide seguence is changedRepressorcannotrecognizeand bindto operator.lacenzymes aresynthesizedconstitutively17-18Fig.15.7
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display 17-18 Changes in the operator DNA to which the repressor Changes in the operator DNA to which the repressor binds can also affect repressor activity binds can also affect repressor activity Fig. 15.7
Theoperon hypothesis:A repressor stops transcription bybinding to an hypotheticaloperator site near promoter of lactose-utilization genes.An inducer can bind to the repressor and prevents it frombinding to the operator.Therepressor (allostericprotein)can change shape whenexposed to inducer.Repressor bound toinducer can'tbindtoDNA.17-19
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display 17-19 The operon hypothesis: hypothesis: A repressor repressor stops transcription by binding to an hypothetical stops transcription by binding to an hypothetical operator operator site near promoter of lactose site near promoter of lactose-utilization genes. utilization genes. An inducer inducer can bind to the repressor and prevents it from can bind to the repressor and prevents it from binding to the operator. binding to the operator. The repressor ( The repressor (allosteric allosteric protein protein) can change shape when ) can change shape when exposed to exposed to inducer inducer. Repressor bound to inducer can . Repressor bound to inducer can’t bind to DNA. to DNA
The operon theory of gene regulationThe operon theory: A single signal can simultaneously regulatethe expression of several genes that are clustered together on achromosome and are involvedin the same biological process.Operon（操纵子）:AunitofDNAcomposedof severalclusteredgenes, plus a promoter and/or operator.These genes areregulated in the same way in response to environmentalchanges and are transcribed into one single mRNAThe genes clustered together on a chromosome are transcribedtogether assinglemRNA(polycistronic)17-20
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display 17-20 The operon theory: A single signal can simultaneously regulate : A single signal can simultaneously regulate the expression of several genes th the expression of several genes that are clustered together on a at are clustered together on a chromosome and are involved in chromosome and are involved in the same biological process. the same biological process. Operon ( Operon 操纵子): A unit of DNA composed of several clustered A unit of DNA composed of several clustered genes, plus a promoter and/or operator. These genes are genes, plus a promoter and/or operator. These genes are regulated in the same way in response to environmental regulated in the same way in response to environmental changes and are transcribe changes and are transcribed into one single mRNA. d into one single mRNA. The genes clustered together on a chromosome are transcribed The genes clustered together on a chromosome are transcribed together as single mRNA ( together as single mRNA (polycistronic polycistronic). The operon theory of gene regulation theory of gene regulation