Positive and Negative Regulation of the β-galactosidaseGene in E. coli
The mechanism of gene expression regulation was first revealed in the β-galactosidasegene*3. This is a fundamental mechanism that controlspromotion and suppression of gene expression.
E. colicannotdirectlyuselactose(seeSelection 4 of Chapter 6, sssFig. 6-7B); however, they canuselactoseafter it is hydrolyzed by their β-galenzyme to produceglucose and galactose.E. colicultured in a medium with glucose do not produce the β-galenzyme (the β-galgene does not function), but in the presence of lactoseinstead of glucose, they uselactose by producing the β-galenzyme.However, the situation is not as simple as mentioned above. The β-galenzyme is not produced when bothglucose and lactose are presenttogether at the same time in the medium.In other words, it is quiteappropriate that the β-galenzyme is synthesizedonlyin the presence of lactose and absence of glucose.
The mechanism for this type of regulation is shown in Fig. 10-1. The operatorsequenceoverlaps the promoter regionupstream of the β-galgene. The action of RNA polymerase is suppressed when a proteincalledrepressor, which is constitutivelyexpressed by i genes,binds to the operator. This is the negative regulation of the β-galgene (Fig. 10-1A). In the presence of lactose, allolactose—the metabolite of lactose—binds to the repressor protein, which then loses its repressorfunction and can no longerbind to the operator.Therefore, if RNA polymerasecanbind to the promoterin the presence of lactose,mRNAcan be synthesized from the β-galgene.
However,regulation is more than just that explained above. cAMP(cyclic AMP) is synthesizedinsidecells. In the lactose operonsystem,RNA polymerasecanbind to the promoteronlyafter the cAMP–CRP complex [(i.e., a complex in which cAMPbinds to CRP*4 (or CAP)) has bound to the promoter. This is the positive regulation of the β-galgene (Fig. 10-1B).
However,evenin the presence of lactose,β-galgene expression is suppressedin the presence of glucose, which is a fairlycomplicatedmechanism.In the presence of a sufficientamount of glucose, the transporter protein that transportslactose into the cell is absolutelyinhibited, and as a result, allolactose is not synthesized and the repressor does not dissociate from the operator.Thus, the β-galgene is not expressed. This mechanism was discovered by Hiroji Aiba et al. of Japan in 1996, 40 yearsafter François Jacob and Jacque Monod’s discovery of regulatory mechanisms.
Gene regulationinvolvespositive and negativeregulatory proteins,each of which binds to the promoter region of a gene,thusregulating its transcription. The base sequences of such regions are shown in Fig. 10-2. Similarregulationmechanismsexist not only for genesinvolved in the utilization of carbohydrates such as arabinose but also for those involved in the metabolism of amino acids and othersubstances.
*3 The β-galactosidasegene was formally referred to as the lacZ gene, but here we have referred to it as β-gal for simplicity.
*4 The prefix“epi”means “over,” “above,” or “outer.”
OperonsRegulatingSimultaneousExpression of Multiple Genes
ColumnFig. 10-1 Structuralcomparison of prokaryotic and eukaryoticmRNA
Escherichia colipossessgenes for enzymesinvolved in the synthesis of all amino acids,carbohydrates,lipids, and nucleic acids from ammonia and glucose, and expression of the genes is regulated as required. As an example, if a culture mediumcontains the amino acidhistidine, then genes for the 10 types of enzymesinvolved in histidinesynthesis are all suppressed, and if the mediumcontains no histidine, then the genes for these 10 enzymes are all simultaneouslyexpressed. In case of the β-galgene, three related genes are also expressed or suppressed at the same time. These multiple genesexist in seriesalongDNA, and mRNA that successivelyreads these genes is synthesized.In other words, one mRNAmoleculecontainsinformation on multiple genes. This type of mRNA is calledpolycistronicmRNA (ColumnFig. 10-1). The term“cistron” is synonymous with genes.
An operon is a functioningunit of multiple genes under the control of a singleregulatory domain(operator) . For example, , lactose and histidineoperons.In general, with regard to synthesis and utilization of nutrients,prokaryotes have purposive regulationmechanisms in which an operonconsisting of many genessynthesizes a polycistronicmRNA.Ribosomesbind to eachcoding region on the polycistronicmRNA,therebysynthesizingproteins.Eukaryotes,however, do not have operons and thus do not producepolycistronicmRNA.
Operons are not the onlymethod for a prokaryote to regulatesimultaneousexpression of multiple genes. As an example, when prokaryotes are exposed to heat shock from an increase in environmentaltemperature,unusualtranscription initiationfactors are produced. These factorsinduceexpression of multiple genes for proteinscalledheat shockproteins that actagainst the heat shock.Genes for the heat shockproteins do not actuallyform a continuousoperon and are ratherscatteredaround in DNA. The system that regulatessimultaneousexpression of this type of scatteredgenegroup is called a regulon.In other words, this example is a heat shockregulon. In the samemanner, there is an SOSregulon that inducesexpression of many differentDNA repair enzymeswheneverDNA is damaged.
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