Evolutionary origin of B family DNA-dependent DNA polymerases from retrotranscriptases

The evolution of DNA and DNA polymerases was a crucial step in the evolution of life on Earth. In the present work, we reconstruct the ancestral sequence and structure for the B family polymerases. Using comparative analyses, we infer the transient state between the ancestor retrotranscriptase and the contemporary B family DNA polymerases. Exonuclease motif was detected in the primary ancestral sequence, as well as an elongation-functioning motif. It is remarkable that the ancestral molecule is more comparable to the retrotranscriptases in terms of structural domains, even though we discovered similarities in the primary sequence with proteins from the B family of DNA polymerases. The present B family proteins differ structurally from retrotranscriptases the most, although the reconstruction of the ancestor protein was able to capture the transitional steps between these two families of polymerases.