Thermococci-to-ClostridiaPathway for the Evolution of the Bacteria Domain

Abstract

With the identification of an archaeal Last Universal Common Ancestor phylogenetically related toMethanopyrus, 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 clostridialMahella australiensis(Mau) were the oldest bacteria. ClostridialThermincola potens(Tpo) also displayed an elevated similarity VARS-IARS bitscore. Overall, the high-bitscore bacteria dominated byClostridiacomprised a number of hydrogen producers. A search for archaea capable of hydrogen production that might be ancestral to the Bacteria domain yielded candidates led byThermococciwhich, likeClostridia, form hydrogen through dark fermentation. A two-domain VARS tree based onMahella,Thermincola, a broad spectrum of archaea together with both well known and newly reported species ofThermococciallocated these twoClostridiato a minor-Thermococcal division on the tree. The kinship betweenThermoccociandClostridiasuggested by this allocation was substantiated by conserved oligopeptide segments on their VARS sequences. It was therefore suggested that aThermococci-to-Clostridiaevolutionary pathway brought about the emergence of the Bacteria domain.