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Lecture 15

Lecture 15: "Prokaryotic Gene Regulation II"

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Western University
Biology 2581B
Jim Karagiannis

Genetics Lecture No. 15: Prokaryotic Gene Regulation II th Wednesday March 6 , 2013 A Model Of Induction For The Lac Operon: -Based on their genetic analysis, Jacob and Monod developed a simple, yet highly detailed model of induction: 1) A repressor protein (coded for by the lacI gene) represses transcription in the absence of lactose, 2) This repressor protein inhibits transcription by binding to a DNA sequence adjacent to the promoter (operator) and blocking RNA polymerase binding, and 3) Induction results from the binding of the inducer (allolactose) to the repressor, preventing the repressor from binding the operator. Key results reached included: Loss of function mutations in lacI leading to constitutive expression, lacI gene c c products acting in trans, lacO mutations leading to constitutive expression, lacO acting in cis, under specialized circumstances (PaJaMo expt.) induction is possible in the absence of the inducer, and lacI S mutants being non-inducible (unable to bind allolactose). Predictions Raised By The Operon Model: -This hypothetical model gave rise to several testable predictions. 1) The repressor protein must physically bind the operator sequence; this was determined by DNA footprinting experiments establishing the region of DNA to which a protein binds. 2) The repressor protein must physically bind the wild-type operator sequence, but not the lacO mutant; this was determined by attaching a radioactive tag to the lac repressor protein so it can be followed in the experiment. 3) The repressor protein must have two distinct domains, one that binds DNA and one that binds the inducer. LacI - s mutations that affect DNA binding all cluster in the N-terminus of the protein, while LacI mutations affecting inducer binding all cluster in the main body of the protein. The LacI repressor is a tetramer of - four identical lacI encoded subunits that binds DNA through the helix-turn-helix motif of the DNA- binding domain. 4) The conformation of the LacI repressor must be altered by the inducer binding (the repressor is an allosteric protein that undergoes reversible changes in conformation when bound to another molecule. This is shown in the binding of allolactose to the LacI repressor altering the conformation of the DNA-binding domain. The Role Of Glucose In The Control Of Transcription: -Jacob and Monod failed to address the role of glucose in the control of transcription. The presence of the preferred catabolite (glucose) represses transcription of the lacZ gene. This phenomenon is referred to as catabolite repression. E. coli grown with both lactose and glucose do not show full induction of lacZ transcription because lacZ transcription is also dependent on a positive regulatory protein, cAMP receptor protein (CRP) also known as catabolite activator protein (CAP). CRP/CAP promotes the ability of RNA polymerase to initiate transcription. Glucose indirectly controls the activity of CRP/CAP through modulating cAMP levels: If glucose concentration is high (low adenyl cyclase levels), low cAMP levels, CAP does not bind to DNA, low transcription of the
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