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ku, Tokyo I I3Aphidicolin at 2 pg/ml caused 90% inhibition of mitotic cell division of sea urchin embryos at the I-cell stage. However, at 40 ,ug/ml it did not affect meiotic maturational divisions of starfish oocytes, which do not involve DNA replication. At 2 ,ug/ml it caused 90% inhibition of incorpo ration of tritiated thymidine into DNA of sea urchin embryos but did not affect protein or RNA synthesis even at a higher concentration. At 2 pg/ml it also caused 90% inhibition of the activity of DNA polymerase a, obtained from the nuclear fraction of sea urchin embryos, but did not affect the activity of DNA polymerase , B or 7. These findings suggest that DNA polymerase a is responsible for replication of DNA in sea urchin embryos.Much of our knowledge of cellular events governed by DNA replication has been obtained by experiments using inhibitors of cellular DNA synthesis, such as arabinosyl nucleosides, N-hydroxyurea and 5-fluorouracil. However, conclusions drawn from these experiments can hardly be considered as other than suggestive, or corroborative of data obtained by other means, because arabinosyl nucleosides have undesirable effects aside from blocking DNA synthesis, and N-hydroxyurea and 5-fluorouracil do not inhibit DNA synthesis directly, but prevent deoxyribonucleoside triphosphate biosynthesis, thereby depriving DNA polymerases of necessary substrates (5).In an attempt to find chemicals that directly and selectively block DNA synthesis, we have searched for chemicals that prevent mitotic cell division of sea urchin embryos, which requires DNA synthesis (32), but do not prevent meiotic maturational divisions of starfish oocytes, which are independent of DNA synthesis (37). We found that aphidicolin (3, 6), a tetracyclic diterpene-tetrol, is one such compound (1 6). Furthermore, our recent experiments have shown that aphidicolin inhibits the activity of DNA polymerase c1 obtained from regenerating rat liver without affecting the activity of DNA polymerase p or mitochondria1 DNA polymerase (23).The functional role of the nuclear DNA polymerases in the complex process of DNA replication is not known. Correlative studies of DNA synthesis and the level of DNA polymerase activity suggest that DNA polymerase c1 may be important in DNA replication (35).However, in certain cases, DNA polymerases fl and y also increase at the time of DNA synthesis, and attempts to specify which of the DNA polymerases is replicative and to assign roles to the various polymerases have not yet been successful (35). 119

Left pneumonectomy was performed on 4 week-old male Fischer-344 rats. Changes in DNA biosynthesis and the activities of related enzymes were studied in the contralateral lungs of the pneumonectomized animals (n = 55) and compared with sham-operated (n = 55) and untreated control animals (n = 40) The wet weight of the contralateral lung of the pneumonectomized rats reached that of both lungs of the untreated and sham-operated rats 14 days after the operation. The activities of thymidine kinase and DNA polymerase from the regenerating lungs were elevated on Days 1 and 7. To determine the molecular forms of DNA polymerase in the crude extract, phosphocellulose column chromatography was performed. The type of DNA polymerase with the highest activity was alpha in regenerating lung on Days 1, 3, and 7. These results suggest that DNA replication for cellular proliferation was elevated in the remaining lung after pneumonectomy. In addition, an interlobar difference in DNA biosynthesis was observed in the remaining lung. The increase was especially marked in the cardiac lobe, followed by increases in the DNA content of the remaining lobes on Day 7. From these observations we conclude (1) that increased activity of DNA polymerase alpha is likely to be an initial change in compensatory lung growth, and may be caused by some unknown stimulator in lung tissue, and (2) that DNA biosynthesis may differ among the lobes of the lung, at least until 3 days post-pneumonectomy.