Tetrahymena pyriformis W were brought to a nonmultiplying state by removal of required amino acids from their growth medium. After amino-acid replacement, the incorporation rates of H3-uridine, H3-thymidine and H3-leucine were measured by the autoradiographic method. Following amino-acid replacement, the first response was detected in RNA synthesis, then protein synthesis, then DNA synthesis and, lastly, in cell division. Amino-acid deprived cells showed a 23% net increase in DNA content, a result supporting the view of others that protein synthesis is not necessary for the initiation of DNA synthesis but is necessary for the maintainance of DNA synthesis.
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