DNA ligase in Axolotl egg: a model for study of gene activity control

Replacement of the light form of DNA ligase (6 S) by the heavy form (8 S) in activated egg of Axolotl has been studied as a model for change in genetic activity exerted by the female pronucleus. Nuclear transplantation shows that a blastula nucleus is able to govern the replacement of the light ligase by the heavy one. The result is not the same if the grafted nucleus is taken from an androgenetic embryo, devoid of the heavy enzyme. Therefore the change in the properties of the female pronucleus appears stable and autoreproducible. Gamma irradiations delivered at different times after activation establish that the replacement of the ligase forms depends on an intact nucleus up to 3 hr 30 min after activation, and thereafter is achieved independently of any nuclear damage. Inhibitors of DNA replication impede the change of enzymatic form in reversible process, suggesting new chromatin synthesis as prerequisite for expression of the new genetic activity. The quantitative level of DNA ligase activity does not show any dose effect when one or many nuclei are present in the same cytoplasm. However, a change in nucleotide concentration results in a change in DNA ligase activity, indicating cytoplasmic control of enzymatic regulation.