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The mode of action by aphidicolin on DNA polymerase a from the nuclear fraction of seaurchin blastulae was studied. The inhibition of DNA polymerase a by aphidicolin was uncompetitive with activated DNA and competitive with the four deoxynucleoside triphosphates using activated DNA as a template-primer. For truncated (residual or limited) DNA synthesis with only three deoxynucleoside triphosphates, aphidicolin inhibited the residual synthesis more strongly in the absence of dCTP than in the absence of each of the other three deoxynucleoside triphosphates. The inhibition was reversed with excess dCTP but not with the other three deoxynucleoside triphosphates. That is, aphidicolin inhibited DNA polymerase a by competing with dCTP with a K , value of 0.5 pg/ml and by not competing with the other three deoxynucleoside triphosphates. dTMP incorporation with the activated DNA was more sensitive to aphidicolin than dGMP or dTMP incorporation with poly(dC) . (dG)12-~8 or poly(dA) . (dT)12-18. Similar resuIts were obtained for DNA polymerase a (B form) from mouse myeloma MOPC 104E.We have previously reported that aphidicolin, a tetracyclic diterpene-tetraol [1,2], inhibits mitotic division of sea-urchin embryos but not meiotic maturational divisions of starfish oocytes [3,4]. Among the macromolecular syntheses that we have examined in vivo, only DNA synthesis is sensitive to aphidicolin [3]. Of the three DNA polymerase species in seaurchin embryos [6-81, only DNA polymerase x is sensitive to aphidicolin at a dose similar to that which inhibits mitosis. DNA polymerases fl and y are resistant to extremely high doses of aphidicolin [3]. It has been reported that rat liver DNA polymerase a is sensitive to aphidicolin but not DNA polymerases fl and y [5]. From these results, we have concluded that DNA polymerase a is the replicative enzyme [3]. In this paper we report that inhibition of DNA polymerase x activity by aphidicolin is due to competition with dCTP.

A new 20-membered macrolide designated exiguolide has been isolated from the marine sponge Geodia exigua, and its structure determined by interpretation of spectroscopic data. Exiguolide specifically inhibited fertilization of sea urchin (Hemicentrotus pulcherrimus) gametes but not embryogenesis of the fertilized egg.

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