Trp Operon Analysis

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Regulation of trp and ara operon

trp operon: The trp operon is a group of genes that are used, or transcribed, together that codes for the components for production of tryptophan. The trp operon is present in many bacteria, but was first characterized in Escherichia coli. The operon is regulated so that when tryptophan synthesis are not expressed. It was an important experimental system for learning about gene regulation, and is commonly used to teach gene regulation.
Discovered in 1953 by Jacques Monod and colleagues, the trp operon in E.coli was the first repressible operon to be discovered. This operon contains five structural genes: trp E, trp D, trp C, trp B and trp A, which encodes tryptophan synthetase. It also contains a repressive
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The repression system targets the intracellular trp concentration whereas the attenuation responds to the concentration of charged tRNA. Thus, the trpR repressor decreases gene expression by altering the initiation of transcription, while attenuation does so by altering the process of transcription that's already in progress. While the Trp R repressor decreases transcription by a factor of 70, attenuation can further decreases it by a factor of 10, thus allowing accumulated repression of about 700 fold. Attenuation is made possible by the fact that in prokaryotes, the ribosomes…show more content…
Repression: The ara operon is regulated by the AraC protein. If arabinose is absent, the dimer AraC protein represses the structural gene by binding to aral and araO and the DNA forms a loop. The loop prevents RNA polymerase from binding to the promoter of the ara operon, thereby blocking trancription.
Activation: When arabinose is present, arabinose binds AraC and prevents AraC from interacting. This breaks the DNA loop. The two AraC-arabinose complexes bind to the aral1 and aral2 sites which promotes transcription. When arabinose is present, AraC acts as an activator.
If arabinose is present, it builds a complex: AraC + arabinose
This complex is needed for RNA polymerase to bind to the promoter and transcribe the ara operon. Also for activation the binding of another structure to aral is needed: CRP+cyclic AMP so the activation depends on the presence of arabinose and cAMP.
References:
1] William Klug, Cummings, and Spencer: "concepts of genetics" 8th edition. Pearson Education Inc, New Jersey: 2006. pg.394-402.
2] Yanofsky,C (1981): "Attenuation in the control of expression of bacterial operons". Nature 289:

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