The cytidine at the first position of the anticodon (C34) in the AUA codon-specific archaeal tRNA
Ile2 is modified to 2-agmatinylcytidine (agm
2C), which is crucial for the precise decoding of the genetic code. This modification is catalyzed by tRNA
Ile-agm
2C synthetase (TiaS), using ATP and agmatine as substrates. We have determined the crystal structures of TiaS-tRNA
Ile2 complexed with ATP, or with AMPcPP and agmatine, revealing a novel kinase module requisite for activating C34 by phosphorylation. These structures showed that agmatine is essential for placing C34 in the active site. We also revealed the molecular mechanism by which TiaS discriminates tRNA
Ile2 from other tRNAs.
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