Preferential synthesis of low-molecular-weight RNA in uv-irradiated plasma of Physarum polycephalum

Mitotically synchronous surface plasmodia of Physarum polycephalum were irradiated during the G2 phase with a Philips 15-W germicidal lamp. At different intervals after irradiation, the plasmodia were pulse-labeled with [3H]uridine, and RNA was extracted and analyzed on linear sucrose gradients. The radioactivity profiles of the RNA showed that irradiated plasmodia synthesize preferentially low-molecular-weight RNA types, including 4 SRNA, during the delay period prior to the first postirradiation mitosis and during the following short mitotic cycle. Double-labeling experiments, employing [14C]uridine-prelabeled plasmodia which were pulse-labeled with [3H]uridine after irradiation, confirmed this finding. It is also seen that there is an overall reduction in the rate of synthesis of rRNA in the irradiated plasmodia