Laboratoire de Chimie Bactérienne, Université de Provence, Aix-Marseille I, CNRS UPR9043, IFR88, Marseille, France. celine.brochier@ibsm.cnrs-mrs.fr
DNA topoisomerase IB (TopoIB) was thought for a long time to be a eukaryotic specific enzyme. A shorter version was then found in viruses and later on in several bacteria, but not in archaea. Here, we show that a eukaryotic-like TopoIB is present in the recently sequenced genomes of two archaea of the newly proposed phylum Thaumarchaeota. Phylogenetic analyses suggest that a TopoIB was present in the last common ancestor of Archaea and Eucarya. This finding indicates that the last common ancestor of Archaea and Eucarya may have harboured a DNA genome.