A separate proofreading subunit coevolved with variant PHP domains. The trees were constructed using 50 selected sequences from our 47-sequence alignment of C-family DNA polymerases and 72 exonuclease sequences. Numbers indicate the GenInfo Identifier of the polymerase sequences. Two clades, corresponding to (1) α-, β- or γ-proteobacteria and (2) Thermus aquaticus and Aquifex aeolicus are shaded light orange and light grey in both trees, respectively. The tree in (B) shows whether the species to which the polymerase sequence corresponds contains an E. coli-like DNA polymerase III ϵ subunit homologue or not.