DNA polymerase delta of Physarum polycephalum

DNA polymerase delta from the phylogenetically ancient slime mold Physarum polycephalum has been 380-fold enriched from amoebae. It was found to have the properties typical for this type of DNA polymerase from higher eukaryotes with regard to effectors, template-primer acceptance, co-purification with 3'-5'-exonuclease activity, as well as the effect of endogenous proliferating cell nuclear antigen (PCNA) from amoebae on the stimulation and processivity of DNA synthesis. An identified cDNA fragment shows 65.5% identical amino acides with DNA polymerase delta from Saccharomyces pombe. The molecular mass of the polymerase is 125 kDa while that of PCNA is 35 kDa. During size-exclusion chromatography, the highly purified polymerase eluted in the position of 125 kDa, suggesting that no other proteins were tightly complexed with the enzyme. The DNA polymerases from the (mononucleate) amoebae and from the (multinucleate) plasmodia of P. polycephalum have very similar properties in contrast to their differences in phenotype and their mode of nuclear division. The polymerase shows a higher degree of similarity than DNA polymerase alpha, and especially the beta-like DNA polymerase, with the corresponding polymerases of higher eukaryotes. According to antibody staining, DNA polymerase delta is readily fragmented by proteases, even in the presence of inhibitor cocktails. Including freshly prepared cell lysates, proteolytic fragments are reproducible, the most abundant being 50 kDa in size. The DNA polymerase is recognized by the antisera against two peptides which have been derived by PCR-screening of plasmodial cDNA. One of the proteolytic splitting sites is located within an eight amino-acid stretch between the two antigenic sequences.