LEC14 - lac operon is also under positive control...

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lac operon is also under positive control Observation: Lactose media: -gal activity high Glucose media: -gal activity low Glu+lac media: -gal activity low lac repressor is not binding to operator and yet -gal is not being expressed
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Lac operon Combination of negative and positive control lac repressor acts as a negative regulator by turning off transcription CAP-cAMP complex acts as a positive regulator by turning on expression Both regulatory proteins are under allosteric control
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Catabolite Repression (misnomer-actually activation) Glucose is favored over lactose (or any other disaccharide) Level of glucose determines cAMP levels (adenylcyclase is inhibited by glucose) glucose cAMP glucose cAMP
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Formation of cAMP from ATP ATP cAMP glucose
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CAP undergoes a conformation change upon binding to cAMP, allowing it to bind DNA Sharma…Steitz. PNAS 2009;106:16604-16609 ©2009 by National Academy of Sciences No DNA binding DNA binding
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CAP ( C atabolite A ctivator P rotein) aka CRP ( c AMP r eceptor p rotein) CAP-cAMP binds to specific DNA sequences Binding results in a bend in the DNA CAP-cAMP also interacts directly with RNA polymerase to stimulate binding to weak promoter sequences DNA recognition helix cAMP
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X-ray structure: CTD - CAP•cAMP - DNA CTD CAP-cAMP
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CAP-cAMP -61 II I 70
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  • Winter '12
  • Lin
  • DNA, rna polymerase, Pol II, d. low glucose, A. High glucose

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