Abstract
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The aminoacyl-tRNA synthetases (EC:6.1.1.) 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; these synthetases are further divided into three subclasses, a, b and c, according to sequence homology. The synthetases specific for alanine, asparagine, aspartic acid, glycine, histidine, lysine, phenylalanine, proline, serine, and threonine belong to class-II synthetases [7]. This entry represents the N-terminal region of the beta subunit of glycyl-tRNA synthases (class IIc).
The aminoacyl-tRNA synthetases (EC:6.1.1) 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 and are mostly monomeric, while class II aminoacyl-tRNA synthetases share an anti-parallel beta-sheet formation, flanked by alpha-helices [3], and are mostly dimeric or multimeric. 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 aci, 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].
The 10 class I synthetases are considered to have in common the catalytic domain structure based on the Rossmann fold, which is totally different from the class II catalytic domain structure. The class I synthetases are further divided into three subclasses, a, b and c, according to sequence homology. No conserved structural features for tRNA recognition by class I synthetases have been established.
Class-II tRNA synthetases do not share a high degree of similarity, however at least three conserved regions are present [4, 5, 6].
In eubacteria, glycyl-tRNA synthetase (EC:6.1.1.14) is an alpha2/beta2 tetramer composed of 2 different subunits [8, 9, 10]. In some eubacteria, in archaea and eukaryota, glycyl-tRNA synthetase is an alpha2 dimer (see IPR002315), this family. It belongs to class IIc and is one of the most complex synthetases. What is most interesting
is the lack of similarity between the two types: divergence at the sequence
level is so great that it is impossible to infer descent from common genes.
The alpha (see IPR002310) and beta subunits also lack significant sequence similarity.
However, they are translated from a single mRNA [8], and a single chain
glycyl-tRNA synthetase from Chlamydia trachomatis has been found to have
significant similarity with both domains, suggesting divergence from a
single polypeptide chain [10].
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