DNA-ligase activity in eukaryotic cells is carried out by two different molecular forms of the enzyme. This molecular duality, first described in mammals (Söderhäll & Lindhal, 1973), has been reported in chicken (David, 1977), amphibians (Carré, Signoret, Lefresne & David, 1981), fish, sea urchin and crab (Signoret & David, 1986). Depending on the developmental stage and the tissue considered, types of cell are available that express exclusively either one or the other of the two DNA-ligases with a defined level of activity. DNA-ligase I and DNA-ligase II, encoded by distinct structural genes (Thiebaud et al. 1985), can be unambiguously characterized and the advantages offered by the early amphibian embryo have made possible the identification of some basic features of their regulation.

The Gene for DNA-ligase I exists in two Regulated States

The heavy molecular form of the enzyme, referred to as DNA-ligase I, is generally present in proliferating tissues.

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