A recurrent general RNA binding domain appended to plant methionyl-tRNA synthetase acts as a cis-acting cofactor for aminoacylation

The cDNA encoding rice methionyl-tRNA synthetase was isolated. The protein exhibited a C-terminal polypeptide appended to a classical MetRS domain. Th is supplementary domain is related to endothelial monocyte activating polyp eptide II (EMAPII), a cytokine produced in mammals after cleavage of p43, a component of the multisynthetase complex. It is also related to Arc1p and Trbp111, two tRNA binding proteins. We expressed rice MetRS and a derivativ e with a deletion of its EMAPII-like domain. Band-shift analysis showed tha t this extra-domain provides MetRS with non-specific tRNA binding propertie s. The EMAPII-like domain contributed a 10-fold decrease in K-M for tRNA in the aminoacylation reaction catalyzed by the native enzyme, as compared wi th the C-terminally truncated MetRS. Consequently, the EMAPII domain provid es MetRS with a better catalytic efficiency at the free tRNA concentration prevailing in vivo. This domain binds the acceptor minihelix of tRNA(Met) a nd facilitates its aminoacylation. These results suggest that the EMAPII mo dule could be a relic of an ancient tRNA binding domain that was incorporat ed into primordial synthetases for aminoacylation of RNA minihelices taken as the ancestor of modern tRNA.