Specific regulatory role of phosphorylation of calf thymus DNA-topoisomerase I smaller forms on the relaxational activity expression. Phosphorylation role on Topo I smaller forms activity.

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Calf thymus Topo I is found to be associated with three active breakdown products, resolved from intact enzyme, which do not appear to be unique to one extraction procedure. They are phosphoproteins, whose enzymatic activity can be modulated through changes in phosphorylation, and which can be phosphorylated 'in vitro', by N II protein kinase, in the same five sites as the intact enzyme. Different amounts of 32P incorporated are observed however, in the corresponding sites. We conclude: 1. proteolysis is probably an 'in vivo' phenomenon, as the Topo I smaller species are observed, during isolation from the earlier crude fractions, and as a minimum of them is always present, even if precautions are taken to minimize proteolysis; 2. a specific regulatory role in the DNA relaxational activity might be played by N II protein kinase phosphorylation, indeed, in the smaller species; 3. the different degrees of 32P incorporation, in analogous phosphorylation sites, might represent a different signal for modulating the gene expression.