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Summary The gene encoding the DNA topoisomerase I from the thermophilic bacterium Fervidobacterium islandicum has been cloned by using oligonucleotides derived from internal microsequencing of the purified protein ( Bouthier de la Tour et al., 1993). The corresponding nucleotide sequence has been determined, revealing an open reading frame of 2097 nucleotides. The deduced amino acid sequence (699 residues) clearly showed that the gene belongs to E. coli TopA family, and has 53% identity with Thermotoga maritima TopA ( Bouthier de la Tour et al., 1995). Comparison of the F. islandicum sequence to the topoisomerase I sequences of other species revealed several interesting features: (1) a putative N-terminal tail of 33 amino acids, whose function is unknown. (2) a 8 amino acids insertion close to the active site tyrosine, shared by 3 other bacterial TopA sequences. (3) a unique putative zinc finger that is conserved in 7 out of 9 sequences. However, no significant feature linked to thermophily could distinguish the two thermotogales topoisomerases from their mesophilic counterparts. Finally, the analysis of the gene organization in the vicinity of the TopA locus in different bacterial species showed that it was not conserved.

Summary We review our present knowledge on DNA topoisomerase and DNA polymerase evolution, with emphasis on information obtained by studying these enzymes in Archaea. Two archaeal DNA topoisomerase genes have been sequenced: the reverse gyrase from Sulfolobus acidocaldarius turns out to be a completely novel type of enzyme, likely originating from the fusion of a helicase and a type I DNA topoisomerase, whereas a novobiocin sensitive type II DNA topoisomerase from Haloferax is closely related to bacterial DNA gyrases. Beside reverse gyrase, we recently isolated from Sulfolobus shibatae a type II DNA topoisomerase which has no gyrase activity and exhibits the pattern of drug sensitivity specific for its eukaryotic counterpart. Several archaeal DNA polymerase genes have now been sequenced: they all belong to the DNA polymerase B family, together with the three eucaryal DNA replicases and Escherichia coli DNA pol II, a repair enzyme. All DNA polymerases from family B described so far are sensitive to aphidicolin. We present recent data suggesting that archaeal DNA polymerases resistant to aphidicolin also belong to family B. Phylogenetic trees of DNA topoisomerases and DNA polymerases turn out to be all noncongruent with the rRNA tree and exhibit different and unusual topologies. We discuss several hypotheses which can explain this observation and their implications concerning the nature of the last common ancestor to the three domains. We conclude that this universal ancestor was probably an advanced member of the DNA world that already contained several DNA polymerases and DNA topoisomerases.