Trna synthetases play a key role in protein biosynthesis by catalyzing the specific aminoacylation of trna at their 3' ends, a two step reaction termed "charging" of trna. the tutorial depicts the binding of glutaminyl-trna synthetase and trna®ln. a uracil (u35) provides a key element for the recognition of trnagln by the synthetase enzyme. u35 of the trna is anchored in place by specific amino acids in the synthetase protein interacting ironically with the nearby backbone phosphates. a. amino acids with polar uncharged side chains b. amino acids with negatively charged side chains c. amino acids with positively charged side chains d. amino acids with hydrophobic side chains