RNA synthesis in the mitotic cycle of physarum polycephalum

The rate of uptake of [³H]uridine into RNA has been studied in synchronous cultures of Physarum polycephalum. RNA synthesis was reduced at mitosis and again in mid-interphase, and sharply increased following each of these periods. The maximum increase early in interphase coincided with the time of nucleolar reconstruction and DNA synthesis, while the second maximum occured at the time of nucleolar swelling. Such swelling is the first cytological indication of the approaching mitosis. Pretreatment with actinomycin D reduced the postmitotic synthesis one half and completely eliminated premitotic synthesis. These results suggest a different physical or chemical nature of some of the RNA's made during the two periods of synthesis.