ROLE OF CALF THYMUS DNA-TOPOISOMERASE-I PHOSPHORYLATION ON RELAXATION ACTIVITY EXPRESSION AND ON DNA-PROTEIN INTERACTION - ROLE OF DNA-TOPOISOMERASE-I PHOSPHORYLATION

Calf thymus DNA-Topoisomerase I activity was found to be altered by changing in phosphorylation: it was completely inhibited upon dephosphorylation by alkaline phosphatase, but incubation with N II protein kinase and ATP restored the relaxation activity to a level higher than that observed prior to dephosphorylation. The calf thymus Topoisomerase I-mediated DNA cleavage, induced by camptothecin, also proved to be inhibited by dephosphorylation, which, apparently, stabilizes the initial enzymesubstrate complex. We conclude that:- the native protein is partially phosphorylated, - the phosphorylation involvement is essential for the activity expression and also for DNA-protein interaction, - changes in the degree of phosphorylation might be involved in the regulation of DNA processing; that evokes some properties of chromatinic peptide models, which bind DNA only when phosphorylated and leads to the assumption that they represent the minimum functional substrate for N II protein kinase. © 1990 Kluwer Academic Publishers.