the lac operon – an inducible operon

Click here to load reader

Post on 07-Feb-2016

42 views

Category:

Documents

0 download

Embed Size (px)

DESCRIPTION

The lac operon – An inducible operon. Genes are either cis (coupling) acting Genes affect those adjacent to themselves Operator and structural genes Or trans (repulsion) acting Genes affect other genes that are not adjacent to themselves Implies the action of a diffusible product - PowerPoint PPT Presentation

TRANSCRIPT

  • The lac operon An inducible operonGenes are either cis (coupling) actingGenes affect those adjacent to themselvesOperator and structural genes

    Or

    trans (repulsion) actingGenes affect other genes that are not adjacent to themselvesImplies the action of a diffusible productRegulator genes

  • Trans (repulsion) acting genesGenes that affect other genesAre not adjacent to the genes they controlRegulator genesImplies the action of a diffusible product

  • Genes that are cis (coupling) actingGenes affect genes adjacent to themselvesOperator and structural genes

  • The lac operon An inducible operonBacteria are monoploid one copy of each gene

    Can make bacteria partially diploid (two copies of a gene)Place another copy of the gene(s) on a plasmid (F)

    E.coli lac I mutant with a plasmid carrying a functional repressor gene will be designated: I - O + Z +/F I +

  • The lac operon An inducible operon

  • The lac operon An inducible operon

  • The lac operon An inducible operon

  • The lac operon An inducible operon

  • The lac operon An inducible operon

  • plasmidE. Coli genome

  • The lac operon An inducible operon

  • The lac operon An inducible operon

  • The lac operon An inducible operon

  • The lac operon An inducible operonIn F plasmid, an entire lac operon can be supplied. For example,

    I + O + Z - Y +/F I - O + Z + Y -

    What are the beta-galactosidase and permease activities with and without lactose in the E. coli strain with the above genotype?

  • The lac operonIf E. coli cells are provided with both glucose and lactose, it preferentially metabolizes glucose rather than lactose. How do E. coli cells accomplish this?

    Positive control and catabolite repression

  • Positive Control and Catabolite RepressionNew Players:Cyclic AMP (cAMP)CAP (catabolite activator protein)

  • High glucose low cAMP

    Low glucose high cAMPPositive Control and Catabolite Repression

  • High glucose low cAMP

    Low glucose high cAMPPositive Control and Catabolite Repression

  • High glucose low cAMP

    Low glucose high cAMPPositive Control and Catabolite Repression

  • The cAMP-CAP complex interacts with the promoter(neither can bind the promoter of lac operon by themselves)CAP = Catabolite Activator Protein

  • Facilitates the attachment of the RNA polymerase to the promoter acts as an activatorMust be a cAMP-CAP complex to bind to the promoter

  • Positive Control and Catabolite RepressionWhat would happen to the lac operon when:Glucose present (+), but lactose absent (-)

  • (low)No transcriptionThe cAMP-CAP complex is not formed (low cAMP), so CAP does not bind to the promoterIn addition, there is no inducer (lactose), so the active repressor is bound to the operator, and therefore the RNA polymerase cannot bind and transcribe the Lac operon genes

  • Positive Control and Catabolite RepressionWhat would happen to the lac operon when:Glucose present (+), but lactose absent (-)No transcription

  • Positive Control and Catabolite RepressionWhat would happen to the lac operon when:Glucose present (+), but lactose absent (-)No transcriptionGlucose and lactose both present (+)

  • The cAMP-CAP complex does not form (low cAMP)The inducer (lactose) is present and inactivates the repressorThere is some transcription, but it is inefficient (no cAMP-CAP complex)

  • Positive Control and Catabolite RepressionWhat would happen to the lac operon when:Glucose present (+), but lactose absent (-)No transcriptionGlucose and lactose both present (+)Very little transcription

  • Positive Control and Catabolite RepressionWhat would happen to the lac operon when:Glucose present (+), but lactose absent (-)No transcriptionGlucose and lactose both present (+)Very little transcriptionGlucose absent (-), but lactose present (+)

  • The cAMP-CAP complex does form (high cAMP), facilitating the attachment of the RNA polymerase to the promoterThe RNA polymerase is effectively attached to the promoter, enhancing transcription

  • Positive Control and Catabolite RepressionWhat would happen to the lac operon when:Glucose present (+), but lactose absent (-)No transcriptionGlucose and lactose both present (+)Very little transcriptionGlucose absent (-), but lactose present (+) A lot of transcription

  • Positive Control and Catabolite RepressionWhat would happen to the lac operon when:Glucose present (+), but lactose absent (-)No transcriptionGlucose and lactose both present (+)Very little transcriptionGlucose absent (-), but lactose present (+)A lot of transcriptionGlucose and lactose both absent (-)

  • No inducer (no lactose) No transcriptionRepressor stays bound to the operator cAMP pairs with CAP and can attach to the promoter

  • Positive Control and Catabolite RepressionWhat would happen to the lac operon when:Glucose present (+), but lactose absent (-)No transcriptionGlucose and lactose both present (+)Very little transcriptionGlucose absent (-), but lactose present (+)A lot of transcriptionGlucose and lactose both absent (-)No transcription

  • Lac operon overview - animationhttp://highered.mcgraw-hill.com/sites/0072556781/student_view0/chapter12/animation_quiz_4.html