DNA polymerase activity (DPA) was investigated in a synchronous culture of the unicellular green alga, Chlorella ellipsoidea (Tamiya's strain). During the algal cell cycle, which was observed in the synchronous growth, nuclear DNA (nDNA) is synthesized near the end of the light phase, whereas the major chloroplast DNA (chDNAM) is synthesized in the middle preceeding the synthesis of nDNA (1). The DPA on a per cell basis correlated well with the synthetic patterns of the cellular DNAs, except

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... ing the early stages. The two marked rises in total cellular DPA, thus corresponded directly to the syntheses of the two DNA species. Fractionation of DNA-depleted enzyme preparations by DEAE-cellulose column chromatography showed two kinds of DNA polymerases. One was eluted at 0.13 M-KCl (Pol I) and the other at 0.15 M-KCl (Pol II). Pol II activity (DPA-II) reached its maximum during chDNA synthesis, whereas Pol I activity (DPA-I) reached its maximum at the end of nDNA synthesis period. We concluded that Pol II synthesizes or replicates chDNA in vivo and Pol I, nDNA; hence, we have designated them "chloroplastic" and "nuclear" DNA polymerases. We also examined the inhibitory effects of chloramphenicol (CP) and cycloheximide (CH) on the development of the two DPAs during the cell cycle. DPA-II was inhibited principally by CP, and DPA-I by CH. Moreover, CH somewhat inhibited DPA-II development during the early phase, and CP greatly inhibited DPA-I development after chDNA synthesis. Thus, Chlorella cells contain at least two kinds of DNA polymerases, chloroplastic and nuclear. The former probably is synthesized on chloroplastic ribosomes and the latter on cytoplasmic ribosomes. Chlorella cells usually contain one nucleus, one chloroplast and one mitochondrion (2). A repetitive synchronous culture of the strain, C. ellipsoidea (Tamiya's strain), has been established for some time (3). We have used it, as a model system for green