The aminoacyl-tRNA synthetase (also known as aminoacyl-tRNA ligase) catalyse the attachment of an amino acid to its cognate transfer RNA molecule in a highly specific two-step reaction [1,2]. These proteins differ widely in size and oligomeric state, and have limited sequence homology [3]. The 20 aminoacyl-tRNA synthetases are divided into two classes, I and II. Class I aminoacyl-tRNA synthetases contain a characteristic Rossman fold catalytic domain and are mostly monomeric [4]. Class II aminoacyl-tRNA synthetases share an anti-parallel beta-sheet fold flanked by alpha-helices [5], and are mostly dimeric or multimeric, containing at least three conserved regions [6, 7, 8]. However, tRNA binding involves an alpha-helical structure that is conserved between class I and class II synthetases. In reactions catalysed by the class I aminoacyl-tRNA synthetases, the aminoacyl group is coupled to the 2'-hydroxyl of the tRNA, while, in class II reactions, the 3'-hydroxyl site is preferred. The synthetases specific for arginine, cysteine, glutamic acid, glutamine, isoleucine, leucine, methionine, tyrosine, tryptophan and valine belong to class I synthetases. The synthetases specific for alanine, asparagine, aspartic acid, glycine, histidine, lysine, phenylalanine, proline, serine, and threonine belong to class-II synthetases. Based on their mode of binding to the tRNA acceptor stem, both classes of tRNA synthetases have been subdivided into three subclasses, designated 1a, 1b, 1c and 2a, 2b, 2c [9].

This entry recognises all class-II enzymes except for heterodimeric glycyl-tRNA synthetases IPR006194 and alanyl- tRNA synthetases.

1. Aminoacyl-tRNA synthetases, the genetic code, and the evolutionary process. Microbiol. Mol. Biol. Rev. 64, 202-36
2. Aminoacyl-tRNA synthetases: versatile players in the changing theater of translation. RNA 8, 1363-72
3. Partition of tRNA synthetases into two classes based on mutually exclusive sets of sequence motifs. Nature 347, 203-6
4. The 2.0 A crystal structure of Thermus thermophilus methionyl-tRNA synthetase reveals two RNA-binding modules. Structure 8, 197-208
5. Structural basis for transfer RNA aminoacylation by Escherichia coli glutaminyl-tRNA synthetase. Biochemistry 32, 8758-71
6. The aminoacyl-tRNA synthetase family: modules at work. Bioessays 15, 675-87
7. Classes of aminoacyl-tRNA synthetases and the establishment of the genetic code. Trends Biochem. Sci. 16, 1-3
8. Sequence, structural and evolutionary relationships between class 2 aminoacyl-tRNA synthetases. Nucleic Acids Res. 19, 3489-98
9. Evolution of aminoacyl-tRNA synthetases--analysis of unique domain architectures and phylogenetic trees reveals a complex history of horizontal gene transfer events. Genome Res. 9, 689-710

The aminoacyl-tRNA synthetase (also known as aminoacyl-tRNA ligase) catalyse the attachment of an
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