One polypeptide with two aminoacyl-tRNA synthetase activities

The genome sequences of certain archaea do not contain recognizable cystein yl-transfer RNA (tRNA) synthetases, which are essential for messenger RNA-e ncoded protein synthesis. However, a single cysteinyl-tRNA synthetase activ ity was detected and purified from one such organism, Methanococcus jannasc hii. The amino-terminal sequence of this protein corresponded to the predic ted sequence of prolyl-tRNA synthetase. Biochemical and genetic analyses in dicated that this archaeal form of prolyl-tRNA synthetase can synthesize bo th cysteinyl-tRNA(Cys) and prolyl-tRNA(Pro). The ability of one enzyme to p rovide two aminoacyl-tRNAs for protein synthesis raises questions about con cepts of substrate specificity in protein synthesis and may provide insight s into the evolutionary origins of this process.