DNA polymerase mu (Pol μ), homologous to TdT, could act as a DNA mutator in eukaryotic cells

A novel DNA polymerase has been identified in human cells. Human DNA polymerase mu (Pol μ), consisting of 494 amino acids, has 41% identity to terminal deoxynucleotidyltransferase (TdT). Human Pol μ, overproduced in Escherichia coli in a soluble form and purified to homogeneity, displays intrinsic terminal deoxynucleotidyltransferase activity and a strong preference for activating Mn2+ ions. Interestingly, unlike TdT, the catalytic efficiency of polymerization carried out by Pol μ was enhanced by the presence of a template strand. Using activating Mg2+ ions, template-enhanced polymerization was also template-directed, leading to the preferred insertion of complementary nucleotides, although with low discrimination values. In the presence of Mn2+ ions, template-enhanced polymerization produced a random insertion of nucleotides. Northern-blotting and in situ analysis showed a preferential expression of Pol μ mRNA in peripheral lymphoid tissues. Moreover, a large proportion of the human expressed sequence tags corresponding to Pol μ, present in the databases, derived from germinal center B cells. Therefore, Pol μ is a good candidate to be the mutator polymerase responsible for somatic hypermutation of immunoglobulin genes.This work was supported by grants from DGES (PB97-1192) and CAM (08.1/0044/98) to L.B.; CAM (08.1/0044.2/98) to A.B.; and by an institutional grant from Fundación Ramón Areces to Centro de Biología Molecular ‘Severo Ochoa’. The Department of Immunology and Oncology (CNB) was founded and supported by Pharmacia and Upjohn, and Consejo Superior de Investigaciones Científicas. J.F.R. was a predoctoral fellow from the Ministerio de Educación y Ciencia. T.K., T.L.d.L. and M.G.-D. were postdoctoral fellows from the Comunidad de Madrid