Regulation of Gene Synthesis 



455 



The preceding evidence indicates that in 

 vivo transcription of genetic DNA can pro- 

 duce genetic RNA. We have already noted 

 that RNA can produce DNA by transcrip- 

 tion in vitro (p. 289). The Rous sarcoma 

 virus (RSV) infects chick embryo cells; this 

 is an RNA virus, and infectious RNA can 

 be isolated from Rous sarcoma cells. When 

 DNA synthesis is inhibited soon after ex- 

 posure of cells to RSV, the production of 

 progeny virus is prevented; if the inhibition 

 occurs later, however, virus progeny are pro- 

 duced. RNA-DNA hybridization experi- 



ments reveal that upon infection with RSV, 

 the chick cell synthesizes DNA homologous 

 to the viral RNA. 14 This DNA is not pres- 

 ent before infection and is not homologous 

 to RNA unrelated to RSV RNA. It has 

 been suggested that this new DNA is the 

 provirus stage of RSV and is comparable to 

 the prophage stage of lambda. This is ap- 

 parently a case of in vivo transcription from 

 genetic RNA to genetic DNA, and perhaps 

 also of transcription in the reverse direction. 



14 See H. M. Temin (1964). 



SUMMARY AND CONCLUSIONS 



Some of the biochemical pathways leading to the synthesis of bacterial and phage 

 DNA are outlined. These pathways involve a large number of specific enzymes. Since 

 enzymes are directly specified by gene action, we have gained some insight into the 

 genetic control of gene synthesis. The occurrence in DNA of bases other than A, T, 

 C, and G has given, or is expected to give, further insight into this matter. Further 

 investigation of factors which determine polynucleotide composition, length, and single- 

 or double-strandedness are also needed before we can fully understand how genetic 

 nucleic acids are regulated in vivo. 



In vivo, genetic DNA can be transcribed to genetic RNA (metagons) and genetic 

 RNA (Rous sarcoma virus) can be transcribed into apparently-genetic DNA. 



REFERENCES 



Bessman, M. J., "The Replication of DNA in Cell-Free Systems," Chap. I, pp. 1-64, 

 in Molecular Genetics, Part I, Taylor, J. H. (Ed.), New York: Academic Press, 

 1963. 



Cairns, J., "The Chromosome of Escherichia coli," Cold Spring Harb. Sympos. Quant. 

 Biol., 28:43-47, 1964. 



Cohen, S. S., "On Biochemical Variability and Innovation," Science, 139:1017-1026, 

 1963. 



Ellis, D. B., and Paranchych, W., "Synthesis of Ribonucleic Acid and Protein in Bac- 

 teria Infected with an RNA Bacteriophage," J. Cell. Comp., Physiol., 62:207-213, 

 1963. 



Fong, P., "The Replication of the DNA Molecule," Proc. Nat. Acad. Sci., U.S.. 52: 

 641-647, 1964. 



Gibson, I., and Beale, G. H., "The Action of Ribonuclease and 8-Azaguanine on Mate- 

 Killer Paramecia," Genet. Res. (Camb.), 4:42-54, 1963. 



Gibson, I., and Sonneborn, T. M., "Is the Metagon an m-RNA in Paramecium and a 

 Virus in Didinium?," Proc. Nat. Acad. Sci., U.S., 52:869-876, 1964. 



Holland, J. J., "Depression of Host-Controlled RNA Synthesis in Human Cells During 

 Poliovirus Infection," Proc. Nat. Acad. Sci., U.S., 49:23-28, 1963. 



