Lack of sugar discrimination by human Pol µ requires a single glycine residue

DNA polymerase mu (Pol m) 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 m is not able to discriminate against the 2¢-OH group of the sugar moiety. It inserts rNTPs with an efficiency that is 1000-fold discrimination characteristic of most DNA-dependent DNA polymerases. The lack of sugar discrimination by Pol m 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 m based on the available Pol b 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 m was mutated to the consensus tyrosine present in Pol b, 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 m in DNA repair.