The deoxyribonucleic acid polymerases from the diatom Cylindrotheca fusiformis. Subcellular distribution, exonuclease activity and heterogeneity of the enzymes

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Abstract
Four DNA polymerases [EC 2.7.7.7] from the marine diatom C. fusiformis, polymerases A, B, C and D, were further differentiated by their subcellular localization, presence of [DNAse] activity, apparent heterogeneity and MW. Polymerases A, B and D occur in significant amounts in the soluble fraction, suggesting that they were originally localized in the nuclei, whereas polymerase C predominates in the chloroplasts. A mitochondrial DNA polymerase was also isolated and characterized by ion-exchange chromatography. Polymerase D has an associated nuclease activity which prefers denatured DNA and Mg2+, and has a pH optimum higher than that for polymerase activity. Co-elution from a DEAE-Sephadex column and co-sedimentation in glycerol density gradients of DNase and polymerase D activity suggest a molecular association. Polymerases A, B and C are devoid of nuclease activity. Glycerol-gradient-sedimentation analysis showed that all DNA polymerase fractions are heterogeneous at low ionic strengths with the appearance of a single homogeneous activity at 0.5 M KCl. Estimated MW of 100,000, 82,000 and 120,000 for polymerases A, B and C, respectively, were obtained from sedimentation analysis and gel filtration. Polymerase D was estimated to have a MW of about 100,000 as determined by sedimentation analysis alone.