Inhibition of DNA synthesis in Ehrlich ascites cells by actinomycin D. II. The presynthetic block in the cell cycle.

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Abstract
Mice bearing intraperitoneal Ehrlich ascites tumors were injected every 2 hr with actinomycin D (0.016 ug/gm of body weight). A 50% inhibition of RNA (ribonucleic acid) synthesis was evident within 30 min after the first injection of actinomycin D, whereas DNA (deoxyribo-nucleic acid) synthesis was not affected for 270 min and reached a maximum inhibition only after 510 min. Autoradiographic studies indicated that actinomycin D, at this low level, inhibited a step in the G1 phase of the cell cycle, preventing the initiation, but not affecting the continuation, of DNA biosynthesis. The activity of thymi-dine kinase and DNA polymerase was not affected under these conditions. Sucrose gradient analysis indicated that actinomycin D had inhibited the synthesis of ribosomal RNA. The results can be interpreted as indicating that in the G1 phase of the Ehrlich ascites cells is an actinomycin-sensitive step whose inhibition prevents the entrance of cells into the DNA synthesis phase.