De Novo DNA Synthesis by Human DNA Polymerase λ, DNA Polymerase μ and Terminal Deoxyribonucleotidyl Transferase

DNA polymerases (pols) catalyse the synthesis of DNA. This reaction requires a primer-template DNA in order to grow from the 3′OH end of the primer along the template. On the other hand terminal deoxyribonucleotidyl transferase (TdT) catalyses the addition of nucleotides at the 3′OH end of a DNA strand, without the need of a template. Pol λ and pol μ are ubiquitous enzymes, possess both DNA polymerase and terminal deoxyribonucleotidyl transferase activities and belong to pol X family, together with pol β and TdT. Here we show that pol λ, pol μ and TdT, all possess the ability to synthesise in vitro short fragments of DNA in the absence of a primer-template or even a primer or a template in the reaction. The DNA synthesised de novo by pol λ, pol μ and TdT appears to have an unusual structure. Furthermore we found that the amino acid Phe506 of pol λ is essential for the de novo synthesis. This novel catalytic activity might be related to the proposed functions of these three pol X family members in DNA repair and DNA recombination.