Molecular Base of Transsulfursome for Cys-tRNA^Cys Synthesis in Indirect Pathway

Abstract

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.