Practice question

PylRS pyrrolysyl-tRNA synthetase encoded by pylS gene is class IIc aminoacyl-tRNA synthetase responsible for specifically charging tRNAPyl with pyrrolysine and synthetic analogues. Unlike most synthetases recognizing anticodon nucleotides, PylRS identity relies on acceptor stem features, discriminator base, and variable region, permitting retention of amber anticodon CUA required for suppression. Active site harbors deep hydrophobic cavity accommodating bulky pyrroline ring while excluding lysine via steric and electrostatic discrimination. ATP activates lysine carboxylate as adenylate transferred to tRNAPyl A76. Orthogonal PylRS-tRNAPyl pair functions in diverse hosts without cross-reacting, forming foundation for genetic code expansion technology enabling site-specific noncanonical amino acid insertion efficiently.