Thermus thermophilus Contains an Eubacterial and an Archaebacterial Aspartyl-tRNA Synthetase^†,‡

Thermus thermophilus possesses two aspartyl-tRNA synthetases (AspRSs), AspRS1 and AspRS2, encoded by distinct genes. Alignment of the protein sequences with AspRSs of other origins reveals that AspRS1 possesses the structural features of eubacterial AspRSs, whereas AspRS2 is structurally related to the archaebacterial AspRSs. The structural dissimilarity between the two thermophilic AspRSs is correlated with functional divergences. AspRS1 aspartylates tRNA^Asp whereas AspRS2 aspartylates tRNA^Asp, and tRNA^Asn with similar efficiencies. Since Asp bound on tRNA^Asn is converted into Asn by a tRNA-dependent aspartate amidotransferase, AspRS2 is involved in Asn-tRNA^Asn formation. These properties relate functionally AspRS2 to archaebacterial AspRSs. The structural basis of the dual specificity of T. thermophilus tRNA^Asn was investigated by comparing its sequence with those of tRNA^Asp and tRNA^Asn of strict specificity. It is shown that the thermophilic tRNA^Asn contains the elements defining asparagine identity in Escherichia coli, part of which being also the major elements of aspartate identity, whereas minor elements of this identity are missing. The structural context that permits expression of aspartate and asparagine identities by tRNA^Asn and how AspRS2 accommodates tRNA^Asp and tRNA^Asn will be discussed. This work establishes a distinct structure−function relationship of eubacterial and archaebacterial AspRSs. The structural and functional properties of the two thermophilic AspRSs will be discussed in the context of the modern and primitive pathways of tRNA aspartylation and asparaginylation and related to the phylogenetic connexion of T. thermophilus to eubacteria and archaebacteria.