Question

to identify genes coding for essential proteins, researchers can create temperature - sensitive mutations. these mutations allow proper protein folding and cell proliferation at the permissive temperature of 22°c, but they cause protein misfolding and reduced cell proliferation at a higher restrictive temperature, such as 37°c. which of the following mutations might increase protein flexibility and lead to a temperature - sensitive phenotype? amino acid side chain amino acid side chain aspartic acid asp d negatively charged alanine ala a nonpolar glutamic acid glu e negatively charged glycine gly g nonpolar arginine arg r positively charged valine val v nonpolar lysine lys k positively charged leucine leu l nonpolar histidine his h positively charged isoleucine ile i nonpolar asparagine asn n uncharged polar proline pro p nonpolar glutamine gln q uncharged polar phenylalanine phe f nonpolar serine ser s uncharged polar methionine met m nonpolar threonine thr t uncharged polar tryptophan trp w nonpolar tyrosine tyr y uncharged polar cysteine cys c nonpolar polar amino acids nonpolar amino acids choose one or more: a. mutation of an alanine to a cysteine, leading to the formation of a new disulfide bond b. a premature stop codon that truncates a protein 10 amino acids from the amino terminus c. mutation of a lysine (that was involved in an ionic bond with a glutamic acid) to a glycine d. mutation of a bulky isoleucine that was buried in the protein interior to a glycine (side chain = h)

Explanation:

1. Option A: Formation of a new disulfide bond would increase protein stability and decrease flexibility, so it's incorrect.
2. Option B: A premature stop - codon truncating the protein near the amino - terminus would likely disrupt the overall protein structure severely rather than just increasing flexibility, so it's incorrect.
3. Option C: Changing a lysine involved in an ionic bond to glycine would disrupt a stabilizing interaction. But glycine's small size can increase flexibility. However, the disruption of an ionic bond is a significant change in the protein's electrostatic interactions.
4. Option D: Changing a bulky isoleucine buried in the protein interior to glycine (with a very small side - chain) would reduce steric hindrance and increase the local flexibility of the protein, which is consistent with a temperature - sensitive phenotype as it can affect protein folding at different temperatures.

Answer:

D. mutation of a bulky isoleucine that was buried in the protein interior to a glycine (side chain = H)