A naturally occurring mini-alanyl-tRNA synthetase.

Alanyl-tRNA synthetase (AlaRS) retains a conserved prototype structure throughout its biology, consisting of catalytic, tRNA-recognition, editing, and C-Ala domains. The catalytic and tRNA-recognition domains catalyze aminoacylation, the editing domain hydrolyzes mischarged tRNA Ala , and C-Ala-the major tRNA-binding module-targets the elbow of the L-shaped tRNA Ala . Interestingly, a mini-AlaRS lacking the editing and C-Ala domains is recovered from the Tupanvirus of the amoeba Acanthamoeba castellanii. Here we show that Tupanvirus AlaRS (TuAlaRS) is phylogenetically related to its host's AlaRS. Despite lacking the conserved amino acid residues responsible for recognition of the identity element of tRNA Ala (G3:U70), TuAlaRS still specifically recognized G3:U70-containing tRNA Ala . In addition, despite lacking C-Ala, TuAlaRS robustly binds and charges micro Ala (an RNA substrate corresponding to the acceptor stem of tRNA Ala ) as well as tRNA Ala , indicating that TuAlaRS exclusively targets the acceptor stem. Moreover, this mini-AlaRS could functionally substitute for yeast AlaRS in vivo. This study suggests that TuAlaRS has developed a new tRNA-binding mode to compensate for the loss of C-Ala.