38. BIOSYNTHESIS OF PROTEINS IN BACTERIAL CELLS 427 



peptide chain can adhere to each ribosome, only 3^4 of the newly synthe- 

 sized peptides will be found to be still adhering to the ribosomes after -1 

 minutes of exposure to the radioactive amino acid. 



Accordingly, the negative results of Roberts et al. bl are not conclusive, 

 and new experiments with shorter times of exposure to the tracer are re- 

 quired. 



c. The Cell Membrane as the Protein-Forming Site 



It has been observed 116 ' 117 that during growth of B. megaterium or E. 

 coli, radioactive amino acids are incorporated more rapidly into the protein 

 fraction of the membrane than into the cytoplasmic proteins. The fraction 

 considered here as cytoplasmic membranes corresponds actually to the 

 "ghosts" described originally by Weibull, 48 i.e., the insoluble fraction found 

 after osmotic lysis of protoplasts. These experiments have led to the con- 

 clusion that cytoplasmic membranes could be the active site of protein 

 synthesis. 



In addition to these in vivo experiments it has been shown that amongst 

 the subcellular fractions of bacteria, membrane preparations are by far 

 the most active in incorporating amino acids into protein. 117 " 120 Enzyme and 

 RNA synthesis have also been reported in the membranes. 117, 118 



Butler et a/.. 116 observe that the proteins manufactured in the mem- 

 brane diffuse very rapidly into the cytoplasm. Thus, labeled preparations 

 of membranes, when mixed with unlabeled cytoplasmic fractions, transfer 

 in 1 minute 60 % of their radioactive proteins to the cytoplasm. No transfer 

 occurs in the absence of cytoplasm. 



Although the incorporation of amino acid into the protein fraction of 

 isolated membranes is stimulated by ribonuclease and not by the sRNA 

 and the activating enzymes, 117 • 120 it must be remembered that the prepara- 

 tions still contain appreciable amounts of RNA and some activating sys- 

 tems. For instance, in Spiegelman's experiments, the amino acid incorpora- 

 tion in vitro depends on the presence of adenosine triphosphate (ATP) 117 ; 

 furthermore, formation of amino acid RNA complexes has been observed in 

 the preparations used by the British authors. 121 



The hypothesis that bacterial membranes could be the primary site of 

 protein synthesis is perhaps premature since they still contain ribonucleo- 



116 J. A. V. Butler, A. R. Crathorn, and G. D. Hunter, Biochem. J. 69, 544 (1958). 



117 S. Spiegelman, 7th Intern. Congr. Microbiol., Stockholm Symposium No. 2, p. 81 

 (1958). 



118 E. F. Gale and J. P. Folkes, Biochem. J. 59, 661 (1955). 



119 B. Nisman, Biochim. et Biophys. Acta 32, 18 (1959). 



120 B. Nisman and H. Fukuhara, Compt. rend. acad. sci. 248, 1438 (1959). 



121 G. D. Hunter, P. Bzookes, A. R. Crathorn, and J. A. V. Butler, Biochem. J. 73, 

 369 (1959). 



