GENETIC CONTROL OF CELL INTEGRATION 311 



tein. Systems of this sort are already being developed for the in vitro 

 synthesis of hemoglobin, of tryptophane synthetase, and of /3-galactosidase. 



In addition to the soluble proteins of the cell sap and the ribosomal 

 proteins themselves, evidence has been found of protein synthesis in the 

 nucleus. 



Protein synthesis in the nucleus has been studied in cells of the thymus 

 gland — thvmocvtes — in which the nucleus constitutes 65 per cent of the 

 dry weight of the cell. Whether the synthetic processes found in these 

 specialized cells also occur in other cell types has not yet been estab- 

 lished. In the thymocytes, protein synthesis has been studied both with 

 intact cells and with isolated nuclei, by following the incorporation of 

 radioactive amino acids into protein. Amino acid-activating enzymes 

 and s-RNA have been implicated in the pathway, which appears similar 

 to that of cytoplasmic protein. A remarkable distinction of the nuclear 

 system is its reported insensitivity to ribonuclease, an enzyme which 

 instantaneously stops ribosomal protein synthesis by degrading 5-RNA. 



Very recent in vitro studies of protein synthesis have demonstrated that 

 the incorporation of C^^ labeled amino acids into protein requires the 

 presence of ribosomes, of s-RNA, and of the amino acid activating enzymes 

 plus an energy-generating system to make the amino acid-s-RNA com- 

 plexes, as well as unidentified enzymes and cofactors present in the super- 

 natant fraction from which the ribosomes have been sedimented. Spec- 

 tacular results have been achieved with synthetic polynucleotides, such 

 as poly-uridylic acid, which increase the incorporation rates several 

 thousand fold. The added polynucleotides act as templates for protein 

 synthesis, and provide a direct means for examining the coding relations 

 between nucleic acids and proteins. (See Postscript.) 



NUCLEIC ACID SYNTHESIS 



Our current understanding of the mechanism of DNA synthesis comes 

 primarily from the studies by Kornberg and co-workers of the in vitro 

 synthesis of DNA by an enzyme preparation from E. coli. As discussed 

 in Chapter 7, the in vitro system requires the presence of all four of 

 the deoxyriboside triphosphates and of a pre-existing DNA "primer," as 

 well as the enzyme. The newly formed DNA exhibits the base ratios of 

 the primer, indicating that the primer and not the enzyme determines the 

 composition of the new polynucleotide. With the recent synthesis of a 

 polynucleotide containing only deoxvadenylate and thymidylate, it has 

 been possible to test the primer hypothesis critically. Using the A-T 

 copolymer as primer, the newly synthesized material was found to con- 

 tain only the adenine- and thymine-containing nucleotides. 



