engleski

Homologs of unusual methanogenic-type seryl-tRNA synthetases

Seryl-tRNA synthetases (SerRS) are essential enzymes which catalyze synthesis of seryl-tRNASer, needed for protein biosynthesis. Serine is activated using ATP, and transferred to cognate tRNA. These enzymes are evolutionarily well preserved and comprise a tRNA-binding domain attached to the catalytic core responsible for serine activation and consequent tRNA serylation. Only methanogenic archaea possess highly diverged, methanogenic-type SerRS enzymes, whose 3D structure and mechanism of substrate recognition is substantially different. However, in some eubacterial genomes, a putative open reading frame homologous to the methanogenic-type SerRS lacking tRNA-binding domain can be found, together with canonical bacterial-type SerRS. We have cloned putative open reading frames homologous to catalytic core of methanogenic-type SerRS from Agrobacterium tumefaciens and Bradyrhizobium japonicum. Curiously, two hypothetical ORFs for homologs of methanogenic-type SerRS can be found in B. japonicum genome. Recombinant proteins were overexpressed in E. coli and purified. Preliminary biochemical characterization has shown that these proteins are homodimeric and they contain zinc, as expected from homology to methanogenic-type SerRS. Surprisingly, homolog from A. tumefaciens preferentially activates alanine, while preferred substrate for both B. japonicum homologs is glycine. Serine is activated by A. tumefaciens homolog, although with lower affinity in comparison with alanine, while serine is not the substrate for B. japonicum homologs. Enzymes display prominent deviations from Michaels-Menten kinetics with preferred amino acids. Serine activated by homolog from A. tumefaciens is not transferred to cognate tRNA, while tRNA-aminoacylation activity for B. japonicum homologs and other amino acids remains to be tested. Switch in amino acid specificity was not anticipated, since substrate selectivity of aminoacyl-tRNA synthetases is usually evolutionary strongly preserved. The final acceptor(s) of activated amino acids remain to be determined. Biochemical and biological role of these enzymes is currently unknown. Since there are few documented cases of aminoacyl-tRNA synthetase (aaRS) duplications, homologs may represent atypical aaRS lacking (yet unidentified) tRNA-binding domain provided in trans. On the other hand, these enzymes may be adopted for biological process unrelated to protein biosynthesis since surveyed organisms do possess canonical aaRS.

seryl-tRNA synthetase; amino acid activation

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