GENETIC CONTROL OF CELL INTEGRATION 



309 



cytosine equal to guanine. In addition, a number of unusual bases, such 

 as pseudouracil, thymine, 2-methyladenine, 6-methylaminopurine, and 

 1-methylguanine have been found in this fraction. The terminal nucleo- 

 tide sequence has been reported to be the same in each molecule, con- 

 sisting of amino acid-adenylic acid-cytidylic acid-cytidylic acid-RNA. If 

 this is correct, the specificity must reside interior to this sequence. It now 

 seems probable that the same enzyme catalyzes both steps 1 and 2. 



3. The activated amino acids are transferred from the s-RNA fraction 

 to the ribosomes, which are the site of actual protein synthesis in the 

 sense of polymerization of the polypeptide chain. Subsequently, the 

 proteins are released from the particles into the cell sap. The inter- 

 mediate reactions between amino acid-s-RNA complex formation and 

 appearance of a new protein have not yet been resolved. 



In recent kinetic studies with rapidly growing intact cells of E. coli, it 

 was demonstrated, by following the incorporation of S"^^ into protein, 

 that the entire process of protein synthesis could occur in a time interval 

 of less than 5 seconds (Figure 11.3). Furthermore, at least 50 per cent of 

 the total cellular protein synthesis proceeded through the ribosomes. 



Comparisons between mammalian and bacterial systems are par- 

 ticularly interesting because of the apparent differences between them 

 in cytoplasmic organization (see Chapters 7 and 9). Bacteria appear 

 to have no nucleolus, no nuclear envelope, and no respiratory ATP- 



4 8 12 16 20 



(a) 



Fraction Number 

 (b) 



(0 



FIGURE 1 1 .3. Flow chart showing the speed of protein synthesi 

 (E. coil) were treated with radioactive sulphate (S ) then frozen, 

 mented in an ultracentrifuge. Solid lines show distribution of RNA 

 between 8 to 16 on the abscissa represents ribosomes, and the p>eak 

 of the cell is S-RNA (a) Cells incubated 15 seconds with S^^O^". 

 followed by 15 seconds incubation with S^- "chaser", (c) Same 

 120 seconds incubation with S^- "chaser". Note transfer of 

 ribosomes to non-sedimentary region (from McQuillen, Roberts, and 

 Acad. So'. Wash., 1959, 45:1437). 



s. Intact bacteria 

 broken and sedi- 



fractions; the peak 



from 20 to the top 

 (b) Same as (a) 



as (o) followed by 

 radioactivity from 



Britten, Proc. Natl. 



