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. These proteins differ widely in size and oligomeric state, and have limited sequence homology [1]. 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 [2]. Class II aminoacyl-tRNA synthetases share an anti-parallel beta-sheet fold flanked by alpha-helices [3], and are mostly dimeric or multimeric, containing at least three conserved regions [4, 5, 6]. 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 [7]. 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.

Methionine-tRNA ligase (6.1.1.10) is an alpha 2 dimer. In some species (archaea, eubacteria and eukaryotes) a coding sequence,similar to the C-terminal end of MetRS, is present as an independent gene which is a tRNA binding domain as a dimer. In eubacteria, MetRS can also be split in two sub-classes corresponding to the presence of one or two CXXC domainsspecific to zinc binding. The crystal structures of a number of methionine-tRNA ligases are known [8, 8, 9].

1. Partition of tRNA synthetases into two classes based on mutually exclusive sets of sequence motifs. Nature 347, 203-6
2. The 2.0 A crystal structure of Thermus thermophilus methionyl-tRNA synthetase reveals two RNA-binding modules. Structure 8, 197-208
3. Structural basis for transfer RNA aminoacylation by Escherichia coli glutaminyl-tRNA synthetase. Biochemistry 32, 8758-71
4. The aminoacyl-tRNA synthetase family: modules at work. Bioessays 15, 675-87
5. Classes of aminoacyl-tRNA synthetases and the establishment of the genetic code. Trends Biochem. Sci. 16, 1-3
6. Sequence, structural and evolutionary relationships between class 2 aminoacyl-tRNA synthetases. Nucleic Acids Res. 19, 3489-98
7. List of aminoacyl-tRNA synthetases. null
8. Crystallographic study at 2.5 A resolution of the interaction of methionyl-tRNA synthetase from Escherichia coli with ATP. J. Mol. Biol. 216, 411-24
9. Crystal structure of Escherichia coli methionyl-tRNA synthetase highlights species-specific features. J. Mol. Biol. 294, 1287-97

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