An inverse relation between the rate of cell division and RNA synthesis per cell in developing frog embryos.

Previous work has shown that endoderm cells synthesize more RNA per cell, including nuclear d-RNA, than do dorsal ectoderm-mesoderm cells (Flickinger, Miyagi, Moser & Rollins, 1967; Woodland & Gurdon, 1968). Both groups found lower levels of DNA synthesis in endoderm cells compared to dorsal ectodermmesoderm cells. It has also been found that in developing frog embryos both LiCl and cytosine arabinoside inhibit DNA synthesis and also inhibit RNA synthesis. Since RNA synthesis is inhibited less than DNA synthesis, the levels of RNA synthesis relative to total DNA increase (Flickinger, Miyagi, Moser & Rollins, 1967). A number of investigators have demonstrated that LiCl can cause isolated amphibian gastrula ectoderm to form endoderm and mesoderm (Barth & Barth, 1962; Gebhardt & Nieuwkoop, 1964; Masui, 1966; Ogi, 1961). Such compounds as NaHC03 (Barth & Barth, 1963) and NaCl (Barth, 1966) can induce isolated frog gastrula ectoderm to form neural tissue. The present investigation was undertaken to examine quantitative variations of nucleic acid synthesis in expiants of Rana pipiens embryos undergoing different rates of cell division. Variations in rates of cell division were obtained by treating embryos with LiCl and NaHC03 or by comparing the rapidly dividing dorsal ectoderm-mesoderm cells with the more slowly dividing endoderm cells. The lengths of the Gl5 S and G2 periods of cells from frog embryos cultured in LiCl and NaHC03, and of dorsal ectoderm-mesoderm and endoderm cells was determined by cell counts and autoradiography with [H]thymidine. Correlations were sought between the lengths of Gl5 S and G2, as well as the rates of DNA synthesis per cell, and the amounts of RNA synthesized per cell. In particular, the concept that cells with longer G2 periods can synthesize more RNA per cell was examined.