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Biochem Brandl 3

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Biochemistry 2280A
Eric Ball

Biochem - Lecture 3 - Nov. 11th Trp Promotor (con’t): • at high concentrations o tryptophan, Trp repressor binds the amino acid tryptophan • the Trp repressor-tryptophan complex binds the operator DNA (can only bind when they are together, ie. complex) • only have this complex when tryptophan is high What happens to transcription? • Figure 8.7 • the Trp repressor-tryptophan complex blocks RNA polymerase from the promotor - STOPS TRANSCRIPTION • when tryptophan concentration is low, tryptophan dissociates from the Trp repressor • The Trp repressor no longer binds the Trp operator - open -10 and -35 The Result: • is that RNAP can access the Trp promotor and transcription occurs What is the structural basis? • Trp repressor - monomer • 107 amino residues • 6 helices • alpha helices 4 and 5 make up the helix-turn-helix motif • functions as a dimer • Trp repressor - functions as a dimer • has 2-fold symmetry • helix 5 of each monomer recognizes adjacent major grooves in the operator DNA • tryptophan binding induces a conformational change in the Trp repressor which allows DNA binding General Themes to take from Trp operon: 1. Trp repressor is a site specific DNA binding protein 2. There is a binding site for Trp repressor within the Trp promoter 3. Trp repressor inhibits transcription by blocking access of RNAP to the promotor 4. Trp repressor is responsive to an environmental signal Lac Operaon of E. coli: • the Lac operon contains those genes required for the metabolism of lactose • it is subject to both positive and negative regulation Negative Regulation: • Figure 8.9 • in the absence of lactose, the Lac repressor binds the Lac operator inhibiting access of RNA polymerase to the promotor • in the presence if lactose, Lac repressor binds to lactose and dissociates from the operator Positive Regulation: • positive regulation of the Lac operon is sensitive to the presence/absence of glucose in the cellular environment - indirectly Catabolite Repression: • cells preferentially use glucose as their carbon source • catabolite repression (def’n): the repression of certain sugar-metabolizing genes when glucose is provided as a carbon source • in E. coli catabolite repression results from transcriptional induction that occurs in the absence of glucose Catabolite Activator Protein: • cAMP is the “second messenger” fo
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