DNA polymerase mu (Pol μ) is a novel family x DNA polymerase that has been suggested to play a role in micro-homology mediated joining and repair of double strand breaks. We show here that human Pol μ is not able to discriminate against the 2'-OH group of the sugar moiety. It inserts rNTPs with an efficiency that is <10-fold lower than that of dNTPs, in sharp contrast with the >1000-fold discrimination characteristic of most DNA-dependent DNA polymerases. The lack of sugar discrimination by Pol μ is demonstrated by its ability to add rNTPs to both DNA and RNA primer strands, and to insert both deoxy- and ribonucleotides on growing nucleic acid chains. 3D-modelling of human Pol μ based on the available Pol β and TdT structural information allowed us to predict candidate residues involved in sugar discrimination. Thus, a single amino acid substitution in which Gly433 residue of Pol μ was mutated to the consensus tyrosine present in Pol β, produced a strong increase in the discrimination against ribonucleotides. The unusual capacity to insert both rNTPs and dNTPs will be discussed in the context of the predicted roles of Pol μ in DNA repair.
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