trp operon presentation

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    TryptophanoperonPradeep Kumar FBT 04CIFE- Mumbai

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    WHY REGULATION OF GENE EXPRESSION???

    Transcription and translation are energeticallyexpensive so in prokaryotes many genes arenot expressed under regular circumstances,gene expression is regulated in response toenvironmental stimuli.

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    Introduction

    Trp operon is group of genes that are used, ortranscribed, together that codes for thecomponents for production of tryptophan & itis regulated so that when tryptophan ispresent in the environment, it is not used.

    First characterized in E.coli . in 1953 byJacques Monod.

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    STRUCTURE } STRUCTURAL GENE

    } CONTROL REGION} REGULATOR

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    STRUCTURAL GENE } trp E+trp D: Anthranilate synthetase}

    trpC

    : Indole glycerolphosphate synthetase} trp B+trp A: Tryptophan synthetase

    the end product of these structural geneproduced a nonproteinaceous component ofrepressor called corepressor. In trp operontryptophan serves as corepressor

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    C ontrol region..

    C onsist of..

    } Promoter - RNA polymerase binds here} Operator - regulatory protein binds here} Leader segment - leader and attenuator

    & lies

    between operator and structural gene analternate mode of regulation of geneexpression known as attenuation.

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    Regulator } Located upstream of control region , not

    contiguous with the operon & synthesizeaporepressor which is a proteinaceoussubstance. It forms a constituent ofrepressor for corepressor. When the latter is not available then in proper strength the

    operator is kept switched on by itselfbecause aporepressor is unable to block the working of operator.

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    Regulation of trp operon} It is regulated by two mechanisms :

    1. Feed Back repression or Repressioncontrols the initiation of transcription

    2. Attenuation governs the frequencyof early transcript termination.

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    Feed Back repression} Whenever corepressor accumulates or

    become available from outside source itcombines with aporepressor formsrepressor and blocks the operator thenstructural gene will stop transcription.

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    Attenuation} Is a second mechanism of negative

    feedback in trp operon and fine tuned it by700 folds & first observed by CH ARLESYANOFSKY .

    } It is performed by a attenuator which isspecific regulatory sequence that whentranscribed in RNA forms a hairpin structure to

    stop transcription when certain condition arenot met e.g. when there is high level of trp. inenv. & it lies between the operator DNA andstructural genes .

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    Features} RNA from the attenuator region is called the

    leader transcript.} C ontains 4 regions than may form stem-loop

    structures, with three possible pairings: 1-2,3-4, and 2-3. Attenuator is made made-up ofStem-loop 3-4 followed by 8 Us: this is a typical -independent transcription termination signal i.emRNA can base pair with itself to form hair loopstructure.} Encodes a leader peptide 14 aas, with two

    adjacent trp residues.

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    }

    If sequence 2 & 3base pair theattenuator structurecannot form andtranscription continue.

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    Attenuating transcription or not???} Regulatory sequence 1 is crucial for

    tryptophan-sensitivity mechanism thatdetermines wether sequence 3 pairs withsequence 2 or with sequence4(transcription stop) & formation ofattenuator structure depend on events

    that occur during translation of regulatorysequence 1, which encode a leader sequence of 14 a.a two of which are trp.

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    When trp. C oncentration arehighthen????} trp tRNA will also be high which allows

    translation to proceed rapidly past the twotrp. C odons of sequence of sequence 1 andinto 2 before 3 is synthesized by RNA pol.

    } Due to this Sequence 2 is covered byribosome and unavailable for pairing tosequence 3, when sequence 3 is synthesized

    then finally sequence 3 and 4 forms whichhalts the transcription or Structural genes nottranscribed due to early termination(attenuation).

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    When trp. C oncentration arelowthen?????} Then ribosome stalls at the two trp. C odon

    in sequence 1 because charged trp. tRNAis less available so sequence 2 remainsfree while 3 is synthesized allowing thesetwo sequence to base pair andpermitting transcription to proceed.

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    So we can say that trp operon is a biosynthetic

    operon which is responsible for synthesis of enzymwhich changes chrosmic acid in to tryptophan and iis regulated by repression 10x effect and

    attenuation 70x effect, so together they have 700x regulation effect.

    SUMMARY

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    TH ANK YOU