Question

1. The antibiotic erythromycin disrupts protein synthesis by preventing ribosomal translocation.  It does not affect any other step in protein synthesis.  Suppose you are doing a translation reaction in vitro and you add erythromycin just before the 3^rd bond is about to be made.  You wait a few minutes and look at the ribosome.
   1. The carboxyl end of the amino acid #3 should now be covalently bonded directly to ___________________
   2. Which tRNA will you find in the A site? ___________________________
   3. The tRNA in the A site will have ___________________________________
   4. Draw the ribsome as it exists now.

Answer 1

a. The carboxyl end of the amino acid #3 should now be covalently bonded directly to its tRNA (peptidyl-tRNA).

b. Which tRNA will you find in the A site? Peptidyl-tRNA bound with polypeptide.

As the erythromycin stops translocation of peptidyl-tRNA from A site of the ribosome to P site, the peptidyl-tRNA (with 3 amino acids attached to it as a growing polypeptide chain) is stuck at the A site.

c. The tRNA in the A site will have the growing polypeptide.

d. Draw the ribosome as it exists now.

The P site of the ribosome would have uncharged-tRNA of amino acid 2, while the A site would have charged-tRNA (peptidyl-tRNA) with growing polypeptide attached to it.

AA – Amino acids

tRNA2 – amino acyl-tRNA of second amino acid

tRNA3 – amino acyl-tRNA of third amino acid.