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Gene regulation in bacteria

1Bacterial Operons: Gene Regulation ModelGenes grouped into operons- Promoter to help initiate transcription- Operator: DNA sequence acts as on-off switch- Genes encode metabolic enzymesOperon regulated by repressors and/or activatorsin response to environment.2Types of Regulated GenesConstitutive genes are always expressedTend to be vital for basic cell functions (often called housekeeping genes)

Inducible genes are normally off, but can be turned on when substrate is presentCommon for catabolic enzymes (i.e. for the utilization of particular resources)Repressible genes are normally on, but can be turned off when the end product is abundantCommon for anabolic enzymes3In bacteria, genes are often clustered into operonsOperons have:Several genes for metabolic enzymes One promoter An operator, or control site(on-off switch)

A separate gene that makes a repressor or activator protein that binds to the operatorRO123PP4The trp Operon5 genes: E, D, C, B, ASame order as enzymes for trp synthesis

Controlled by a single promoter and operator5Repressors and Activators are proteins that bind to DNA and control transcription.

Co-repressors and Inducers: small effector molecules that bind to repressors or activators

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Genes of operonProteinOperatorPolypeptides that make upenzymes for tryptophan synthesisRegulatorygeneRNA polymerasePromotertrp operon53mRNAtrpDtrpEtrpCtrpBtrpAtrpRDNAmRNAEDCBAThe trp operon: regulated synthesis of repressible enzymesFigure 18.21a5Tryptophan absent -> repressor inactive -> operon on7

DNAmRNAProteinTryptophan(corepressor)Active repressorNo RNA madeTryptophan present -> repressor active -> operon off. Figure 18.21bActive repressor can bind to operator and block transcription8

Tryptophan changes the shape of the repressor protein so it can bind DNA9The lac operon: regulated synthesis of inducible enzymesFigure 18.22a

DNAmRNAProteinActiverepressorRNApolymeraseNoRNAmadelacZlaclRegulatorygeneOperatorPromoterLactose absent, repressor active, operon off. The lac repressor is innately active, and inthe absence of lactose it switches off the operon by binding to the operator.(a)5310mRNA 5'

DNAmRNAProteinAllolactose(inducer)InactiverepressorlacllaczlacYlacARNApolymerasePermeaseTransacetylase-Galactosidase53(b)Lactose present, repressor inactive, operon on. Allolactose, an isomer of lactose, derepresses the operon by inactivating the repressor. In this way, the enzymes for lactose utilization are induced.mRNA 5lac operonFigure 18.22b11

12Positive Gene RegulationBoth the trp and lac operons involve negative control of genesbecause the operons are switched off by the active form of the repressor protein

Some operons are also subject to positive controlVia a stimulatory activator protein, such as catabolite activator protein (CAP)13Promoter

Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized.If glucose is scarce, the high level of cAMP activates CAP, and the lac operon produces large amounts of mRNA for the lactose pathway.(a)CAP-binding siteOperatorRNApolymerasecan bindand transcribeInactiveCAPActiveCAPcAMPDNAInactive lacrepressorlacllacZFigure 18.23aIn E. coli, when glucose is always the preferred food sourceWhen glucose is scarce, the lac operon is activated by the binding of the catabolite activator protein (CAP)Positive Gene Regulation- CAP14When glucose is abundant, CAP detaches from the lac operon, which prevents RNA polymerase from binding to the promoterFigure 18.23b

(b)Lactose present, glucose present (cAMP level low): little lac mRNA synthesized. When glucose is present, cAMP is scarce, and CAP is unable to stimulate transcription.Inactive lacrepressorInactiveCAPDNARNApolymerasecant bindOperatorlacllacZCAP-binding sitePromoter15