One Polypeptide with Two Aminoacyl-tRNA Synthetase Activities

Abstract

The genome sequences of certain archaea do not contain recognizable cysteinyl–transfer RNA (tRNA) synthetases, which are essential for messenger RNA–encoded protein synthesis. However, a single cysteinyl–tRNA synthetase activity was detected and purified from one such organism, Methanococcus jannaschii . The amino-terminal sequence of this protein corresponded to the predicted sequence of prolyl–tRNA synthetase. Biochemical and genetic analyses indicated that this archaeal form of prolyl–tRNA synthetase can synthesize both cysteinyl-tRNA ^Cys and prolyl-tRNA ^Pro . The ability of one enzyme to provide two aminoacyl-tRNAs for protein synthesis raises questions about concepts of substrate specificity in protein synthesis and may provide insights into the evolutionary origins of this process.