DNA synthesis following gross alterations of the nucleocytoplasmic ratio in the ciliate Stentor coeruleus

Abstract

Incorporation of externally supplied and injected 3H-thymidine into DNA was measured autoradiographically. Starved stentors synthesized no DNA, in contrast to well-fed animals, but replication commenced in some cases if they were fed. Grafting starved and well-fed stentors together rapidly induced DNA synthesis in the starved partner. Suppression of synthesis in the well-fed macronucleus was not observed. Well-fed cytoplasm alone induced DNA synthesis in starved stentors, and starved cytoplasm grafted to starved animals also induced synthesis after a lag. Starved animals with the beaded macronucleus reduced to 2 nodes commenced DNA replication after 6 hr; however, initiation was prevented if the normal nuclear complement was restored before the fourth hour. The macronucleus was required to render starved cytoplasm capable of supporting DNA synthesis, but once potentiated the cytoplasm alone could initiate replication in a starved nucleus. Initiation required RNA synthesis, shown by actinomycin sensitivity. This nucleic acid analysis suggests that decreasing the nucleocytoplasmic ratio elicits RNA synthesis in the remaining macronucleus. The RNA codes for proteins involved in DNA synthesis which are synthesized in the cytoplasm and enter the nucleus to initiate DNA replication.