Increased uptake of thymidine in the activation of sea urchin eggs. III. Effects of aphidicolin

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Abstract
Uptake and phosphorylation of externally supplied [3H]‐thymidine are fully stimulated in fertilized sea urchin eggs exposed to 5.0 μg/ml aphidicolin. As in untreated controls, the rate of uptake in aphidicolin‐treated eggs increases greater than 50‐fold shortly after fertilization, and greater than 85% of the transported thymidine is immediately phosphorylated to the triphosphate. The intracellular levels of [3H]‐thymidine triphosphate (3H‐dTTP) resulting from an external supply of [3H]‐thymidine is therefore equal in aphidicolin‐treated and untreated fertilized eggs. Under the same experimental conditions, the incorporation of externally supplied [3H]‐thymidine into newly synthesized DNA of fertilized eggs is 90% inhibited by exposure to aphidicolin. The full availability of 3H‐dTTP in these eggs further suggests that aphidicolin inhibits specifically at the level of DNA synthesis. This inhibitory effect is proportional to the concentration of aphidicolin between 0 and 5.0 μg/ml. In the continuous presence of 5.0 μg/ml aphidicolin, fertilized eggs fail to undergo mitotic chromosome condensation, nuclear envelope breakdown, and cytokinesis, suggesting a dependent link between these processes and the completion of nuclear DNA synthesis.