Abstract
A light form of DNA ligase (EC 6.5.1.2), the only form present in oocytes of the axolotl (Ambystoma mexicanum), has been shown to be replaced by a heavy form of the enzyme when the egg enters cleavage. This early biochemical event has been assumed to rely on direct nuclear input. Sucrose gradient analysis permits discrimination between enzymes from axolotl and the sharp-ribbed salamander (Pleurodeles waltlii) for both heavy and light enzymatic forms of DNA ligase. Genetic activity of blastula nuclei transplanted in activated cytoplasm has been tested by determination of the enzymatic forms and specific types of DNA ligases when the implanted egg enters cleavage. A blastula nucleus of Pleurodeles in axolotl cytoplasm determines a heavy ligase of the Pleurodeles type. Conversely, a haploid androgenetic nucleus of Pleurodeles in axolotl cytoplasm controls a light ligase of the Pleurodeles type. Reciprocal experiments give homologous results. To our knowledge, this is the earliest nucleus-dependent synthesis revealed in development for any system. The heavy ligase of one species may coexist with the light form of the other species but not with the light form of its own specific type. Inhibition of the production of the heavy form for one genome results in the expression of the light form. We conclude that genetic control of DNA ligase in very early development involves structural genes for heavy and light forms of enzyme, with an exclusion process operating an alternative expression of corresponding genes. This exclusion relationship between nonallelic genes is species specific.