110C LEC 9 Regulation of bacterial gene expression

110C LEC 9 Regulation of bacterial gene expression - BCH...

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BCH 110C LECTURE 9 Regulation of Bacterial Gene Expression Lodish 6 th ed. Chapter 7.1 & Pages 286-287
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Bacteria need to be efficient • Glucose is the most efficient energy source • Glucose goes directly into glycolysis and on to the TCA cycle • Other nutrients require metabolism to prepare them for glycolysis and/or TCA cycle • This metabolism requires enzymes (and the energy to make them) • Cells need to focus on glucose utilization until it is depleted • Then they focus on more “costly” fuels for energy generation • If they do not, another bacteria will, and there will be no progeny left from the less discriminating bacteria
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Lactose is two linked glucose equivalents • Lactose utilization requires a number of inducible enzymes including: β -galactosidase (Lac Z) – Lactose permease (Lac Y; a transmembrane enzyme which transports galactosides into E. coli) – Lac A; a transacetylase (acetylates and detoxifies pyranosides produced by β -galactosidase activity, which are discharged from the cell and cannot reenter due to their acetylation.)
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Biology and genetics of lactose utilization suggested existence of a genetic switch to Jacob and Monod No Glucose Present
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Switch also Responsive to Glucose Molecular Decision Making
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How is the decision made? β -galactosidase RNA polymerase binding site overlaps the repressor binding site Repressor Binding Site TATA Box -35 Promoter Region CAP binding site
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Lac Repressor • Product of the i gene • i gene is adjacent to the Lac promoter • Lac repressor is a homotetramer of a 37 kdal protein • There are ~10 molecules of repressor per cell • Lac repressor binds its specific DNA binding site (the operator ) very strongly (K diss =10 -13 M) • Each subunit can bind 1 molecule of allolactose (lactose is rapidly isomerized to allolactose by β - galactosidase) • Lac repressor + inducer (allolactose) does not bind DNA • [The rest of the lecture will refer to allolactose as lactose for simplicity (most text books do not make a distinction)]
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Lac repressor-operator interactions Lac repressor tetramer not only binds to the +11 operator, but also reversibly to two other, secondary operators in the Lac promoter region
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110C LEC 9 Regulation of bacterial gene expression - BCH...

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