Characterization of SpPol4, a unique X-family DNA polymerase in Schizosaccharomyces pombe

As predicted by the amino acid sequence, the purified protein coded by Schizosaccharomyces pombe SPAC2F7.06c is a DNA polymerase (SpPol4) whose biochemical properties resemble those of other X family (PolX) members. Thus, this new PolX is template-dependent, polymerizes in a distributive manner, lacks a detectable 3'-> 5' proofreading activity and its preferred substrates are small gaps with a 5'-phosphate group. Similarly to Pol mu, SpPol4 can incorporate a ribonucleotide (rNTP) into a primer DNA. However, it is not responsible for the 1-2 rNTPs proposed to be present at the mating-type locus and those necessary for mating-type switching. Unlike Pol mu, SpPol4 lacks terminal deoxynucleotidyltransferase activity and realigns the primer terminus to alternative template bases only under certain sequence contexts and, therefore, it is less error-prone than Pol mu. Nonetheless, the biochemical properties of this gap-filling DNA polymerase are suitable for a possible role of SpPol4 in non-homologous end-joining. Unexpectedly based on sequence analysis, SpPol4 has deoxyribose phosphate lyase activity like Pol beta and Pol lambda, and unlike Pol mu, suggesting also a role of this enzyme in base excision repair. Therefore, SpPol4 is a unique enzyme whose enzymatic properties are hybrid of those described for mammalian Pol beta, Pol lambda and Pol mu.