Light-induced transient increase of the activity of topoisomerase I in plasmodia of Physarum polycephalum

Abstract 1. 1. Activity of topoisomerase I and incorporation of [ 3 H]uridine and [ 14 C]thymidine were monitored during light-induced sporulation of the slime mold Physarum polycephalun . 2. 2. A 4-fold transient increase of topoisomerase I activity but not of [ 3 H]uridine or [ 14 C]thymidine incorporation was observed after 42 hr of illumination with 6 hr impulses. 3. 3. The activity of topoisomerase I did not increase in the absence of light impulses. However, ca 5-fold increase of the activity was observed in dark when 100 μ M dibutyryl-cAMP was administered 12 hr before harvesting of plasmodia. 4. 4. Fluorodeoxyuridine and cycloheximide administered 36 hr after starting of the illumination cancelled the increase of the activity of topoisomerase I. 5. 5. After 7 days of the illumination, when fruiting bodies appeared, the activity of topoisomerase I dropped to about 15% of the initial value.

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