Biochemical and electron microscopic evidence that cell nucleus negatively controls mitochondrial genomic activity in early sea urchin development.

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Abstract
Enucleated halves of sea urchin [Paracentrotus lividus] eggs obtained by centrifugation contain almost all the mitochondrial population of the egg. Removal of the nucleus followed by parthenogenetic activation stimulates the incorporation of [3H]thymidine into the mitochondrial DNA, whereas no such incorporation is observed in activated whole eggs. The block is not the result of a modification in the permeability of the mitochondrial membrane. EM observations demonstrated duplication of mitochondrial DNA molecules in activated enucleated halves. No duplication was found in the mitochondrial DNA from activated whole eggs or from nonactivated enucleated halves. The cell nucleus apparently exerts a negative control on the activity of the mitochondrial genome through some short-lived nuclear substance(s).