LeuRS (leucyl-tRNA synthetase) catalyses the esterification of tRNAsLeu with leucine. This family of enzymes is divided into prokaryotic and eukaryal/archaeal groups according to the presence and position of specific insertions and extensions. In the present study, we investigated the function of LSD1 (leucine-specific domain 1), which is naturally present in eukaryal/archaeal LeuRSs, but absent from prokaryotic LeuRSs. When mutated in their common domain, the eukaryal and archaeal LeuRSs exhibited defects in the first reaction step of amino acid activation with variations of leucine or ATP-binding strength, whereas the tRNA aminoacylation was moderately affected. When the eukaryal extension was mutated, severe tRNA charging defects were observed, suggesting that eukaryotes evolved this LSD1 extension in order to improve the aminoacylation reaction step. The results also showed that the LSD1s from organisms of both groups are dispensable for post-transfer editing. Together, the data provide us with a further understanding of the organization and structure of LeuRS domains.