Correct answer = B. Ubiquitination usually marks old, damaged, or misfolded proteins for destruction by the cytosolic proteasome. There is no known cellular mechanism for repair of damaged proteins.
2.5. Many antimicrobials inhibit translation. Which of the following antimicrobials is correctly paired with its mechanism of action?
A. Erythromycin binds to the 60S ribosomal subunit.
B. Puromycin inactivates elongation factor-2.
C. Streptomycin binds to the 30S ribosomal subunit.
D. Tetracyclines inhibit peptidyltransferase.
Correct answer = C. Streptomycin binds the 30S subunit and inhibits translation initiation. Erythromycin binds the 50S ribosomal subunit (60S denotes a eukaryote) and blocks the tunnel through which the peptide leaves the ribosome. Puromycin has structural similarity to aminoacyl-transfer RNA. It is incorporated into the growing chain, inhibits elongation, and results in premature termination in both prokaryotes and eukaryotes. Tetracyclines bind the 30S ribosomal subunit and block access to the A site, inhibiting elongation.
2.6. Translation of a synthetic polyribonucleotide containing the repeating sequence CAA in a cell-free protein-synthesizing system produces three homopolypeptides: polyglutamine, polyasparagine, and polythreonine. If the codons for glutamine and asparagine are CAA and AAC, respectively, which of the following triplets is the codon for threonine?
A. AAC
B. ACA
C. CAA
D. CAC
E. CCA
Correct answer = B. The synthetic polynucleotide sequence of CAACAACAACAA … could be read by the in vitro protein-synthesizing system starting at the first C, the first A, or the second A (that is, in any one of three reading frames). In the first case, the first triplet codon would be CAA, which codes glutamine; in the second case, the first triplet codon would be AAC, which codes for asparagine; in the last case, the first triplet codon would be ACA, which codes for threonine.
2.7. Which of the following is required for both prokaryotic and eukaryotic protein synthesis?
A. Binding of the small ribosomal subunit to the Shine-Dalgarno sequence
B. Formylated methionyl-transfer (t)RNA
C. Movement of the messenger RNA out of the nucleus and into the cytoplasm
D. Recognition of the 5′-cap by initiation factors
E. Translocation of the peptidyl-tRNA from the A site to the P site
Correct answer = E. In both prokaryotes and eukaryotes, continued translation (elongation) requires movement of the peptidyl-tRNA from the A to the P site to allow the next aminoacyl-tRNA to enter the A site. Only prokaryotes have a Shine-Dalgarno sequence and use formylated methionine and only eukaryotes have a nucleus and co-and posttranscriptionally process their mRNA.
2.8. α1-Antitrypsin (AAT) deficiency can result in emphysema, a lung pathology, because the action of elastase, a serine protease, is unopposed. Deficiency of AAT in the lungs is the consequence of impaired secretion from the liver, the site of its synthesis. Proteins such as AAT that are destined to be secreted are best characterized by which of the following statements?
A. Their synthesis is initiated on the smooth endoplasmic reticulum.
B. They contain a mannose 6-phosphate targeting signal.
C. They always contain methionine as the N-terminal amino acid.
D. They are produced from translation products that have an N-terminal hydrophobic signal sequence.