Phylogenetic trees of the ribonucleotide reductases of Class I and II (A); type II DNA topoisomerase of the A family (B—left),⁶⁰ thymidylate synthases of the ThyA family (B—right) and DNA polymerases of the B family (RNA-primed) (from ref. 5) (C). Viral sequences are indicated in bold. A=Archaea, B=Bacteria, E=Eukarya. Grey double arrows indicate possible ancient transfer between cells and viruses. Black double arrows indicate recent transfers between cells and viruses. These trees were made with the program PROTML, using the quick add search, with the JTT model of amino acid substitution and retention of the 1,000 top-ranking trees. Boostrap values higher than 90% are indicated by filled circle. The scale bar represents the number of substitution per 100 sites for unit branch length.