Sequence divergence of seryl-tRNA synthetases in archaea

The genomic sequences of Methanococcus jannaschii and Methanobacterium thermoautotrophicum contain a structurally uncommon seryl-tRNA synthetase (SerRS) sequence and lack an open reading frame (ORF) for the canonical cysteinyl-tRNA synthetase (CysRS), Therefore, it is not clear if Cys-tRNA(Cys) is formed by direct aminoacylation or by a transformation of serine misacylated to tRNA(Cys). To address this question, we prepared SerRS from two methanogenic archaea and measured the enzymatic properties of these proteins. SerRS 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 as substrate for aminoacylation. Gel shift experiments showed that M. thermoautotrophicum SerRS did not mischarge tRNA(Cys) with serine. This indicates that Cys-tRNA(Cys) is formed by direct acylation in these organisms.