DNA polymerases of Chlorella. II. Characterization to distinguish the two enzymes

Previously, we reported two DNA polymerases (DPols), Pol I and Pol II, in the unicellular green alga, Chlorella (Aoshima, J., Nishimura, T., & Iwamura, T. (1982) Cell Struc. Funct. 7, 71-86). Changes in their activities during the cell cycle either in the normal and drug-inhibited courses indicated that Pol I and Pol II functioned to replicate nuclear and chloroplast DNAs, respectively. In the present work, we have examined their enzymic properties to characterize and distinguish them further. A number of inhibitors commonly used for such studies were also tested to determine their effects and the results were analyzed by use of the simple and useful "Tamiya plot." We have also analyzed the data obtained in inhibitor studies on various eukaryotic DPols in the literature using the Tamiya plot, and the results will be presented elsewhere (Iwamura, T. & Aoshima, J. (1984) J. Biochem. in press). Comparisons of the algal DPols with mammalian enzymes as regards enzymic properties and inhibition modes have led us to conclude that: [1] the algal two DPols are significantly different from each other, despite having many similarities to each other: [2] they are related in properties to any of the three mammalian DPols-alpha, -beta, and -gamma; Pol I (n-DPol) was a little more like alpha than Pol II (ch-DPol), which in turn more akin to gamma. This feature was quantitized by using vectors in a three-dimensional alpha-beta-gamma-space. Another peculiar feature derived from the Tamiya plot of the inhibitions by araCTP and aphidicolin (both being competitive with cCTP) has led us to propose a specific allosteric role of cCTP in the reaction mechanism besides its role as one of the substrates.