Non-discriminating and discriminating aspartyl-tRNA synthetases differ in the anticodon-binding domain.
In most organisms, tRNA aminoacylation is ensured by 20 aminoacyl-tRNA synthetases (aaRSs). In eubacteria, however, synthetases can be duplicated as in Thermus thermophilus, which contains two distinct AspRSs. While AspRS-1 is specific, AspRS-2 is non-discriminating and aspartylates tRNA(Asp) and tRNA(Asn). The structure at 2.3 A resolution of AspRS-2, the first of a non-discriminating synthetase, was solved. It differs from that of AspRS-1 but has resemblance to that of discriminating and archaeal AspRS from Pyrococcus kodakaraensis. The protein presents non-conventional features in its OB-fold anticodon-binding domain, namely the absence of a helix inserted between two beta-strands of this fold and a peculiar L1 loop differing from the large loops known to interact with tRNA(Asp) identity determinant C36 in conventional AspRSs. In AspRS-2, this loop is small and structurally homologous to that in AsnRSs, including conservation of a proline. In discriminating Pyrococcus AspRS, the L1 loop, although small, lacks this proline and is not superimposable with that of AspRS-2 or AsnRS. Its particular status is demonstrated by a loop-exchange experiment that renders the Pyrococcus AspRS non-discriminating.
Charron
Christophe
C
Département Mécanismes et Macromolécules de la Synthèse Protéique et Cristallogenèse, UPR 9002, 15 rue René Descartes, 67084 Strasbourg cedex, France.
Roy
Hervé
H
Blaise
Mickael
M
Giegé
Richard
R
Kern
Daniel
D
D000595:Amino Acid Sequence; D000926:Anticodon; D001226:Aspartate-tRNA Ligase / Q000737:chemistry / Q000378:metabolism*; D001665:Binding Sites; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011487:Protein Conformation; D019711:Pyrococcus / Q000201:enzymology; D017386:Sequence Homology, Amino Acid; D016963:Thermus thermophilus / Q000201:enzymology
D000926:Anticodon; D001226:Aspartate-tRNA Ligase