RNA metabolism in the macronucleus of Euplotes eurystomus during the cell cycle

Abstract

The rate of RNA synthesis in the macronucleus of Euplotes increases two- to four-fold immediately prior to the onset of DNA synthesis and declines rapidly to the early G1 rate shortly after the initiation of DNA synthesis. During the S phase the rate of RNA synthesis continues to decrease slowly. It is clear that the amount of DNA has little if any influence on the rate of RNA synthesis. Increase in macronuclear RNA occurs predominantly if not exclusively during the G1 period and the first part of the S phase. In Euplotes labeled for one cell cycle and subsequently grown on non-radioactive food the content of radioactive RNA drops by 40% during the first cell cycle, reflecting the breakdown of rapid turnover RNA and/or the exit of RNA from the macronucleus. During the subsequent three cell cycles the amount of radioactive nuclear RNA decreases very slowly. The experiments suggest the presence in the macronucleus of a large amount of relatively stable, non-migrating RNA. This interpretation is supported by observations on starving cells in which the radioactive RNA content of the macronucleus declines very slowly over many days. Autoradiographic observations indicated that the rate of loss of RNA from the macronucleus of growing cells is not detectably influenced by the DNA replication bands that pass through the nucleus during the S phase. During amitosis of the macronucleus the amount of labeled nuclear RNA decreases significantly. The sum total of labeled RNA per predivision macronucleus or per average pair of daughter nuclei is significantly greater than the amount in the average dividing nucleus. The data apparently reflect the transient displacement of some RNA to the cytoplasm during amitosis of the macronucleus.