Ribonucleotides and manganese ions improve non-homologous end joining by human Polm

Abstract

HumanDNA polymerasemu (Polk), a family Xmember involved in DNA repair, has both template-directed and terminal transferase (template-independent) activities. In addition to their ability to incorporate untemplated nucleotides, another similarity between Polm and terminal deoxynucleotidyl transferase (TdT) is their promiscuity in using ribonucleotides (NTPs), whose physiological significance is presently unknown. As shown here, Polm can use NTPs instead of deoxynucleotides (dNTPs) during non-homologous end joining (NHEJ) of non-complementary ends, a Polm-specific task. Moreover, a physiological concentration of Mn2+ ions did benefit Polm-mediated NHEJ by improving the efficiency and accuracy of nucleotide insertion. Analysis of different mutations in the 'steric gate' of the active site indicated that Polm is taking advantage of an open active site, valid for selecting alternative activatingmetal ions and nucleotides as substrates. This versatilitywould allowad hoc selection of the most appropriate nucleotide/metal ion combination for individual NHEJ events to gain efficiency without a cost in terms of fidelity, thus widening the spectrum of available solutions to position a discontinuous template strand in proper register for connection. © 2013 The Author(s).