Sequence divergence of seryl-tRNA synthetases in archaea

The genomic sequences of Methanococcus jannaschii and Methanobacterium ther moautotrophicum contain a structurally uncommon seryl-tRNA synthetase (SerR S) sequence and lack an open reading frame (ORF) for the canonical cysteiny l-tRNA synthetase (CysRS), Therefore, it is not clear if Cys-tRNA(Cys) is f ormed by direct aminoacylation or by a transformation of serine misacylated to tRNA(Cys). To address this question, we prepared SerRS from two methano genic archaea and measured the enzymatic properties of these proteins. SerR S was purified from M. thermoautotrophicum; its N-terminal peptide sequence matched the sequence deduced from the relevant ORF in the genomic data of M. thermoautotrophicum and M. jannnschii. In addition, SerRS was expressed from a cloned Methanococcus, maripaludis serS gene. The two enzymes charged serine to their homologous tRNAs and also accepted Escherichia coli tRNA a s substrate for aminoacylation. Gel shift experiments showed that M. thermo autotrophicum SerRS did not mischarge tRNA(Cys) with serine. This indicates that Cys-tRNA(Cys) is formed by direct acylation in these organisms.