3. The activity of myogenin must be carefully controlled lest it trigger premature expression of the gene essential for cell differentiation. The myogenin gene is turned ON in before it is actually needed (and myogenin is expressed), but myogenin is prevented from functioning by phosphorylation of its DNA-binding domain AND by its tight binding to Id, a helix-loop-helix protein that lacks the DNA binding domain.
A. What types of interactions would hold the heterodimer together and you expect them to be the same as those that are necessary for Id binding? Where would you find these bonds/interactions (use the figure).
B. Explain how phosphorylation of the DNA binding domain and dimerization with Id might act to keep myogenin nonfunctional.
Extra credit. Why might this method of protein regulation be preferred over transcriptional control in this case?
4. Aminoacyl-tRNA synthetases attach specific amino acids to their appropriate tRNAs. The synthetase that attaches valine to tRNAval must be able to discriminate valine from threonine. Valyl-tRNA synthetase achieves this discrimination in two steps. In the first, it uses a binding pocket whose contours allow only valine and threonine to bind, but the binding of valine is preferred. This site is responsible for coupling the amino acid to the tRNA. In the second step, the enzyme checks the newly made aminoacyl-tRNA using a second binding site that is very specific for threonine and hydrolyses it from the tRNA. How do you suppose it is that the second binding site can be very specific for threonine, whereas the first binding site has only a moderate specificity for valine?
3. The activity of myogenin must be carefully controlled lest it trigger premature expression of the gene essential for cell differentiation. The myogenin gene is turned ON in before it is actually needed (and myogenin is expressed), but myogenin is prevented from functioning by phosphorylation of its DNA-binding domain AND by its tight binding to Id, a helix-loop-helix protein that lacks the DNA binding domain.
A. What types of interactions would hold the heterodimer together and you expect them to be the same as those that are necessary for Id binding? Where would you find these bonds/interactions (use the figure).
B. Explain how phosphorylation of the DNA binding domain and dimerization with Id might act to keep myogenin nonfunctional.
Extra credit. Why might this method of protein regulation be preferred over transcriptional control in this case?
4. Aminoacyl-tRNA synthetases attach specific amino acids to their appropriate tRNAs. The synthetase that attaches valine to tRNAval must be able to discriminate valine from threonine. Valyl-tRNA synthetase achieves this discrimination in two steps. In the first, it uses a binding pocket whose contours allow only valine and threonine to bind, but the binding of valine is preferred. This site is responsible for coupling the amino acid to the tRNA. In the second step, the enzyme checks the newly made aminoacyl-tRNA using a second binding site that is very specific for threonine and hydrolyses it from the tRNA. How do you suppose it is that the second binding site can be very specific for threonine, whereas the first binding site has only a moderate specificity for valine?
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