We have investigated the transcription of ribosomal RNA in plasmodia of Physarum polycephalum by a combination of pulse-labelling with [3H]uridine and RNA:DNA hybridization. The DNA used for the hybridization was a HindIII restriction fragment (cloned in bacteriophage lambda) of Physarum ribosomal DNA that carries a substantial fraction of the rRNA genes, enabling the ribosomal transcripts in the newly synthesized RNA to be measured. We found that ribosomal RNA constituted about 40% of the pulse-labelled RNA at all times during the synchronous mitotic cycle.

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