Archaea Have Their Own Proofreading Mechanism

A team of Yale biochemists investigated a proofreading mechanism in one-celled organisms from the domain Archaea and found it different, but just as effective, as its counterpart in domains Bacteria and Eukarya (the latter including all plants and humans).  Their work was published online in PNAS July 6.¹
    The particular instance involved the ability to discriminate between two similar amino acids, threonine and serine, on the molecule that connects the amino acid to the transfer RNA (aminoacyl-tRNA synthetase, or aaRS).  Members of Archaea have an enzyme that bears no sequence similarity, but is “functionally conserved” (i.e., does the same thing), to that of the other domains.  The archaeal gene is “unrelated to, and absent from,” bacterial and eukaryotic genomes.  The authors term this an instance of “functional convergence of unrelated domains” that “assures specificity” of the correct amino acid to the tRNA molecule.  This “appears to be the first aaRS found to use two evolutionarily unrelated editing domains,” they state.  “The functional convergence between the two ThrRS editing domains is highlighted by the observation that both depend on an absolutely conserved set of histidine residues for their function.”

¹Korencic et al., “A freestanding proofreading domain is required for protein synthesis quality control in Archaea,” Proceedings of the National Academy of Sciences USA, 10.1073/pnas.0403926101.

For a cell to be able to proofread at all is a profound morsel of food for thought.  How proofreading could emerge by chance is challenge for Darwinian evolution to explain, but for it to have arisen twice by different means is vastly more improbable (see online book).