Autoradiographic studies of the effects of some antibiotics, amino acid analogs, and nucleases on the synthesis of DNA have been carried out to gain information on the regulatory mechanism of DNA replication within the cell. Cultured cells of the Chinese hamster and human cancer (HeLa) were used as experimental material.

Puromycin, chloramphenicol, p-fluoro-phenylalanine, and methylated tryptophan analogs inhibited in varying degree the synthesis of protein. They also affected the synthesis of deoxyribonucleic acid (DNA) by changing the number of cells able to incorporate thymidine-H3 as well as by reducing the rate of thymidine incorporation in the replicating cells. The relationship of histone and DNA synthesis was studied cytochemically, and data obtained indicate that histones do not seem to play any direct role in DNA replication. Actinomycin D inhibited intracellular ribonucleic acid (RNA) synthesis but did not affect DNA synthesis. This suggests that there is no necessary coupling of RNA and DNA syntheses. The nucleases (DNase I, DNase II, and RNase), within a short time after contact with the cells in culture medium, caused an increase in the number of cells able to incorporate thymidine. After 3 hours of treatment both DNase II and RNase resulted in decreases in DNA synthesis.

The results are discussed with regard to the hypothesis that RNA may act as repressor for DNA synthesis and that the repressor activity is regulated by amino acids or proteins.

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A portion of a dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Faculty of Pure Science, Columbia University.

This investigation was supported in part by a grant from the Atomic Energy Commission AT (30-1)-1304 to J. Herbert Taylor and by a Faculty Fellowship from Columbia University.

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