Thermococci-to-Clostridia Pathway for the Evolution of the Bacteria Domain

With the identification of an archaeal Last Universal Common Ancestor phylogenetically related to Methanopyrus, the origin of Bacteria becomes a choice between independent emergence versus descent from Archaea. Recently, use of the similarity between paralogous valyl-tRNA synthetase (VARS) and isoleucyl-tRNA synthetase (IARS) as a measure of the ages of bacteria indicated that an Ancestral Bacterial Cluster centred at clostridial Mahella australiensis (Mau) were the oldest bacteria. Clostridial Thermincola potens (Tpo) also displayed an elevated similarity VARS-IARS bitscore. Overall, the high-bitscore bacteria dominated by Clostridia comprised a number of hydrogen producers. A search for archaea capable of hydrogen production that might be ancestral to the Bacteria domain yielded candidates led by Thermococci which, like Clostridia, form hydrogen through dark fermentation. A two-domain VARS tree based on Mahella, Thermincola, a broad spectrum of archaea together with both well known and newly reported species of Thermococci allocated these two Clostridia to a minor-Thermococcal division on the tree. The kinship between Thermoccoci and Clostridia suggested by this allocation was substantiated by conserved oligopeptide segments on their VARS sequences. It was therefore suggested that a Thermococci-to-Clostridia evolutionary pathway brought about the emergence of the Bacteria domain.