A mutant operator sequence was found that contained a C T mutation that reduced
ID: 164560 • Letter: A
Question
A mutant operator sequence was found that contained a C T mutation that reduced the binding affinity of the operator for its repressor 10,000-fold. This mutant bacterial strain was screened for recovery of regular function at the operon, and several revertants (bacteria displaying normal regulation) were recovered. All of them still had the C T mutation in the operator. All of the revertant bacteria had recovered regulation by a compensatory mutation in the repressor
a) In one revertant, the compensatory mutation in the repressor was Arg Gln. Propose an explanation of this result. To do so, sketch the interaction of the Arg of the wild type repressor and the C-G basepair of the wild type operator, and then sketch the similar interaction between the Gln and the mutant operator (C T; remember that the other base in the basepair also changes to maintain normal Watson-Crick pairing)
b) Another revertant had an Arg Glu mutation in the repressor. Sketch the interaction between the mutant operator and the Glu that explains this result. Hint: consider the protonation state of the Glu side chain.
Please help I'm not sure how to go about sketching the interactiones between the proteins and the operators.
Explanation / Answer
Q.No a
C-T substitution to significantly disrupt the folding of protein, because the shape of Arg and Gln is different from each other. Arg having more bulk than Gln and it can still form the ionic bond with Asp. The three dimensional shape of the protein to largely remain the same. In the mutant protein Arg has been replaced by Gln and this mutant is unable to fold properly, rendering the protein unable to function.
Q.No b
The mutant protein is acidic in nature, whereas the wild type protein has a basic residue. So the mutant protein cannot form the ionic bond with the wild type of proteins. Because, the two acidic residues are repel to each other and thus leads to the changed fold of the mutant protein. Depending on the protein density, one or many substitution leads to the formation of mutant protein to regain the shape, which is close to the wild type of protein.
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