Changes in the levels of DNA and RNA syntheses have been studied in unagitated cultures of Acanthamoeba castellanii during the phases of logarithmic multiplication (LM) and population growth deceleration (PGD). Pulse-labeling experiments show that the rate of DNA synthesis decreases at the same time that DNA per cell is known to drop by 50%. The drop in DNA content has been explained by demonstrating with hydroxyurea that the majority of LM amebas can replicate once when DNA synthesis is inhibited and, therefore, must be in G2, whereas the PGD amebas cannot multiply in the presence of inhibitor and, therefore, must be in G1. The inhibition of DNA synthesis in LM or PGD cells has been shown to induce encystment. The rate of RNA synthesis, as illustrated by pulse-labeling experiments, increases 25% in late LM-early PGD while RNA per cell increases 75%. The rate of synthesis then decreases 65%. The majority of accumulated RNA has been demonstrated to be ribosomal by disc electrophoresis. By using actinomycin D at different stages during the RNA build-up, the ability of the amebas to encyst has been shown to depend on the presence of this RNA. The observations on DNA and RNA are discussed with respect to the occurrence of cysts in the cultures during PGD.

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