While aminoacyl-tRNAs are synthesized by aminoacyl-tRNA synthetase (aaRS), many methanogenic archaea lack Cys-tRNA synthetase, and Cys-tRNA
Cys is synthesized through indirect pathway. It has been known that two enzymes are required in such indirect pathway. PhosphoseryltRNA synthetase (SepRS) attaches phosphoserine (Sep) to tRNA
Cys to form Sep-tRNA
Cys which is subsequently converted to Cys-tRNA
Cys by Sep-tRNA : Cys-tRNA synthase (SepCysS). However, the catalytic efficiency of SepRS and SepCysS
in vitro is very low, indicating the existence of unknown components. Here we present a translation factor SepCysE, essential for methanococcal Cys-tRNA
Cys synthesis, and report a crystal structure of SepCysE in complex with SepCysS. Based on the structure, mutational analysis was carried out. Taken all results together, we propose that the CystRNA
Cys synthesis is performed without release of the intermediate by a multienzyme complex named transsulfursome.
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