Regulation of DNA synthesis and cell division in starved-refed synchronized Tetrahymena pyriformis HSM

Tetrahymena pyriformis HSM was starved for 24 h and then, upon refeeding, cellular events were monitored. Inhibition of DNA synthesis by hydroxyurea demonstrated that about 25 % of the refed cells divide without synthesizing DNA after refeeding, i.e., they were stalled in G2, while the rest (75%) were stalled in G1. Inhibition of RNA synthesis with actinomycin D showed that all cells must synthesize RNA up to the beginning of cytokinesis in order to divide. It was also shown that the synthesis of a portion of this RNA begins at about 140 min after refeeding and is a prerequisite for DNA replication which occurs at 180 min. Transcription of this RNA seems to be initiated at various times after 140 min in different cells. DNA polymerase activity begins to increase just before DNA synthesis occurs. Furthermore, the increase in enzyme activity is prevented by inhibitors of RNA or protein synthesis introduced at 120 min after refeeding, suggesting that: (1) the increase in DNA polymerase activity is necessary for subsequent DNA replication; (2) the RNA synthesis initiated at 140 min after refeeding and required for subsequent DNA synthesis probably includes a messenger RNA coding for DNA polymerase.