Misacylation of tRNA(Lys) with noncognate amino acids by Lysyl-tRNA synthetase

Lysyl-tRNA synthetase (LysRS), a class II enzyme whose major function is to provide Lys-tRNA(Lys) for protein synthesis, also catalyzes aminoacylation of tRNA(Lys) with arginine, threonine, methionine, leucine, alanine, serine, and cysteine. The limited selectivity in the tRNA aminoacylation reaction appears to be due to inefficient editing of some amino acids (Met, Leu, Cys, Ala, Thr) by a pre-transfer mechanism or the absence of editing of other amino acids (Arg and Ser). Purified Arg-tRNA(Lys), Thr-tRNA(Lys), and Met- tRNA(Lys) were essentially not deacylated by LysRS, indicating that the enzyme does not possess a post-transfer editing mechanism. However, LysRS possesses an efficient pretransfer editing mechanism which prevents misacylation of tRNA(Lys) with ornithine. A novel feature of this editing reaction is that omithine lactam is formed by the facile cyclization of ornithyl adenylate.