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

Abstract

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 30!50 proofreading activity and its preferred substrates are small gaps with a 50- phosphate group. Similarly to Polm, 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 Polm, SpPol4 lacks terminal deoxynucleotidyl- transferase activity and realigns the primer terminus to alternative template bases only under certain sequence contexts and, therefore, it is less error- prone than Polm. Nonetheless, the biochemical prop- erties of this gap-filling DNA polymerase are suitable for a possible role of SpPol4 in non-homologous end-joining. Unexpectedly based on sequence ana- lysis, SpPol4 has deoxyribose phosphate lyase activ- ity like Polb and Poll, and unlike Polm, suggesting also a role of this enzyme in base excision repair. Therefore, SpPol4 is a unique enzyme whose enzym- atic properties are hybrid of those described for mammalian Polb, Poll and Polm