37. NUCLEIC ACID AND PROTEIN SYNTHESIS 367 



activation, attachment of activated amino acids to a soluble RNA frac- 

 tion, and the reactions involving the terminal nucleotide end groups of this 

 RNA were brought into focus as integral parts of the over-all scheme. 

 These events will be discussed further in the section dealing with the soluble 

 enzyme-ribonucleic acid system. Zamecnik 96 has recently presented an ex- 

 cellent review of the history of growth of our understanding of the mam- 

 malian incorporation systems briefly outlined here. 



In microorganisms, it has, thus far, not been possible to effect as com- 

 plete a separation of the components of a protein synthetic system as in 

 mammalian and higher plant systems, and the role of ribosomes in the 

 process has been assessed in only a preliminary way. We know from the 

 early studies of Gale and Folkes 97 and Spiegelman (cf. review by Spiegelman 4 ) 

 that the enzymic removal of RNA from disrupted bacterial cell preparations 

 destroys the protein synthetic process. As we have seen, yeast and bacteria 

 contain these particles; indeed, most of their RNA is found in such par- 

 ticles. Two recent reports strongly point to the ribosomes as the initial site 

 of protein synthesis in bacteria. McQuillen et a/., 98 using whole E. coli cells, 

 have shown that S 35 in methionine is rapidly and initially incorporated into 

 the protein of the ribosomes whence it subsequently appears in other 

 fractions. Lamborg" has successfully developed an active cell-free incor- 

 poration system from E. coli cells which is almost identical in its require- 

 ments to the cell-free mammalian systems. Thus Mg ++ , ATP and ATP- 

 generating system, GTP, amino acids, ribosomes, and a soluble enzymatic 

 component are necessary for activity. Initial incorporation is into peptide 

 linkage in the protein of the ribosomes, while the soluble protein acquires 

 label more slowly. The possibility of whole cell contamination has been 

 minimized. However, Spiegelman finds that in E. coli the cell membrane 

 material is much more active than the particles plus soluble fraction both 

 in vivo and in vitro. 100 We shall consider later in somewhat more detail some 

 of the recent observations that implicate the bacterial cell membrane in 

 protein synthesis (see also Chapter 38). 



In the cell-free incorporation systems mentioned thus far, it appears that 

 the synthetic machinery is capable of limited but not qualitatively altered 

 protein synthesis. Finished proteins do not appear to accumulate readily 

 in the nonparticulate milieu. This is in contrast to the in vivo system where 

 in a short time the soluble proteins are heavily labeled after exposure to 



96 P. C. Zamecnik, Harvey Lectures 54, 256 (1960). 



97 E. F. Gale, Harvey Lectures 51, 25 (1957). 



98 K. McQuillen, R. B. Roberts, and R. J. Britten, Proc. Natl. Acad. Sci. U. S. 45, 

 1437 (1959). 



99 M. Lamborg, Federation Proc. 19, 346 (1960). 



100 S. Spiegelman, in "Recent Progress in Microbiology" (G. Tunevall, ed.), p. 81. 

 Almquist and Wiksell, Stockholm, 1959. 



