Question

a. Antibody is prepared to the thr repressor protein isolated from a wild-type strain. Competetive antibody binding assays show that several times more repressor is present in an extract prepared from the derepressed mutant described above than in an extract prepared from the wild-type strain. What is your interpretation of these data? b. The repressor protein requires a pH of 6.1 for optimal binding to its operator in vitro. If the pH goes above pH 8.0, the repressor is unable to bind. What amino acid side chains are implicated? Why is binding reduced in alkaline media? c. What effect would an increase in pH have on cell transcription of the thr operon? Why? d. What percentage of the repressor will bind at pH 6.3? More than 50% or less? Now calculate.

Answer 1

Answer! A cysteine side chain has alsoelectric pH of the 8.31, at pH above 8.3 cystein dalill have negative change. operată ie the DNA and hence it is negatively charged Since negative change of eltis repressh at pH 8 would not to allow ettie binding of ettio repressd to the negativelej charged operats DALA. a klhen we have to increasing the pH value aboue Sithen transcription would be increased, as we have already explained The presence of negative charge of the che prere would not allows the binding of sepressed to the operator DALA. Therefore there lill be no burdle f& the movemen of ittee ena poleymerase on the DNA in be

afsteine Woul on the d) mde than soy. wlüte bind to the operato DNA, becaule ar pH 6-3 change on the represse will be same as that pt at 6.12 Troelectric pH of the Ceystein side chain is 8.3. for all the plt below 8.3 have the same charge. The charge afsteine side chain åters pu pu below 8.3 would be neutral. so, the total charge of the repressol at pH below 8.3 will be decided by protonation and depro- tanate on ostate by other charged amino acide present. At pH above 8.3 cysteine add negative change to the total charge of plotein - Q

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