The phosphorylation of Drosophila melanogaster DNA topoisomerase II by purified casein kinase II was characterized in vitro. Under the conditions used, the kinase incorporated a maximum of 2-3 molecules of phosphate per homodimer oftopoisomerase II. No autophosphorylation of the topoisomerase was observed. The only amino acid residue modified by casein kinase II was serine. Apparent Km and Vm.. values for the phosphorylation reaction were 0.4 ,.M topoisomerase II and 3.3 ,umol of phosphate incorporated per min per mg of kinase, respectively. Phosphorylation stimulated the DNA relaxation activity of topoisomerase II by 3-fold over that of the dephosphorylated enzyme, and the effects of modification could be reversed by treatment with alkaline phosphatase. Therefore, this study demonstrates that posttranslational enzymatic modifications can be used to modulate the interaction between topoisomerase II and DNA.The topology of DNA has a profound influence on how its genetic information is regenerated, rearranged, and expressed in vivo (1, 2). Consequently, the enzymes that affect the topological structure of nucleic acids play a crucial role in controlling the cellular functions of DNA (1-4). One class of enzymes, the type II topoisomerases, catalyzes changes in the topological state of nucleic acids by passing one intact DNA helix through a transient double-stranded break made in a second helix (5-7). These ubiquitous enzymes are essential for the viability of eukaryotic cells (8)(9)(10) and are involved in many aspects of DNA metabolism (1-4), including replication' (11)(12)(13)(14)(15)(16)(17), repair (18), transcription (13), and chromosome segregation (9,10).Despite the importance of type II topoisomerases to eukaryotic organisms, little information exists concerning the physiological regulation of these enzymes. Although their activity is stimulated >10-fold by cell proliferation (19-21, §), nothing is known about the events that control this increase in activity. Clearly, before the cellular functions of topoisomerase II can be completely described, the factors that regulate its activity must be well understood.Recently, the type I topoisomerase from Novikoff hepatoma cells was shown to exist in vivo as a phosphoprotein (22). Moreover, when purified, the enzyme was found to be sensitive to its state of phosphorylation, with higher levels of incorporation yielding increased rates of activity in vitro (23,24). This suggests that post-translational modification may play a role in the regulation of topoisomerases. With this in mind, the effects of phosphorylation on the activity of topoisomerase II have been examined. This paper describes the interaction between topoisomerase II and casein kinase II from Drosophila melanogaster. While topoisomerase II showed no autophosphorylation under the conditions used, kinase-mediated phosphorylation of the enzyme stimulated its DNA relaxation activity. Furthermore, the effects of phosphorylation could be reversed by treatment with alkaline phosphatase. This study, th...

To determine relationships between the hormonal activation of casein kinase II and its phosphorylation state, epidermal

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Pat Ackerman is chair of Milwaukee Women inc., and senior vice president of investor relations, corporate responsibility and sustainability, and treasurer at A.O. Smith Corporation. In this article, Pat presents insights and observations from a recent panel discussion focused on corporate board efforts to navigate the chaos and uncertainty businesses face due to the COVID‐19 pandemic and resulting economic crisis.