Synthesis of actinomycin-insensitive RNA during the first postirradiation mitotic cycle, in the synchronously mitotic plasmodia of Physarum polycephalum.

A sucrose density gradient analysis of 3H-uridine pulse-labelled RNA from the first postirradiation mitotic cycle of Physarum polycephalum shows that all the density classes of RNA synthesized during this period are resistant to the peptide-antibiotic, actinomycin D. In fact, the synthesis is found to be greater in the presence of the drug. The heterogenously sedimenting synthetic activity here may represent a single species of RNA and its precursors or more than one kind of RNA. Further characterization of this RNA is meaningful in view of the actinomycin insensitivity of the postirradiation mitotic cycle itself to this antibiotic.