506 J. BRACKET 



sap PNA nor mitochondrial PNA are affected, while apparently the micro- 

 somes greatly decrease in number in animals on a protein-free diet. 



While there is obviously a good deal of evidence favoring the opinion 

 that cytoplasmic particles, microsomes in particular, are the major sites of 

 protein synthesis in the cell, it would, however, be an exaggeration to 

 believe that they are the only possible site of protein synthesis. Experi- 

 mental work bySzafarz ^ has shown that when the flagellate Pohjtomella 

 is grown on labeled acetate-C* as the sole carbOn source, all cell particles 

 have the same specific radioactivity in their proteins. If, when acetate has 

 become the limiting factor for growth, an excess of ordinary acetate is 

 added, brisk protein synthesis occurs, while the specific radioactivity of 

 the various fractions of course decreases. The smaller the PNA-containing 

 granules are, the faster is the drop in specific radioactivity. This might 

 mean either that proteins of the smaller granules (microsomes) have the 

 highest turnover and that these particles are of special importance in 

 protein synthesis, or that their mass increases more rapidly than does 

 that of the other fractions. That this second explanation is correct is 

 shown by later work by Szafarz. ^ When the flagellate is grown in con- 

 tinuous culture, so that all cell constituants grow at the same rate, the 

 protein turnover is identical in all types of granules. Therefore, in the 

 case of microorganisms kept continuously in .the exponential phase of 

 growth, microsomes cannot be the major source of cytoplasmic proteins; 

 nor can these granules be obligate intermediates in protein synthesis. Such 

 a conclusion is in agreement with the fact reported earlier that, in Aceta- 

 bularia, microsomes or chloroplasts are the most important site of protein 

 synthesis, depending on the precursor used (glycine or CO 2 , respectively). 



3. Biochemical Mechanisms of Protein Synthesis 



The various aspects of this all-important biological and biochemical 

 problem have been the object of many recent reviews (Northrop,"^ Lip- 

 mann,^^^ Fruton,^" Borsook,'^* Linderstr0m-Lang,^^^ Haurowitz,^*"'^^^ 

 Chantrenne,^^^ Brachet,'^^ etc.). We shall discuss here only those aspects of 

 protein synthesis which are linked together with PNA metabolism. 



1'^" D. Szafarz, Arch, internal. Physiol. 60, 196 (1952). 

 I'^b D. Szafarz, Arch, internal. Physiol. 61, 269 (1953). 

 "^ J. H. Northrop, "Crystalline Enzymes," p. 232. Columbia Univ. Press, New York, 



1948. 

 "« F. Lipmann, Federation Proc. 8, 597 (1949). 

 '" J. S. Fruton, Yale J. Biol, and Med. 22, 263 (1950). 

 ''8 H. Borsook, Physiol. Revs. 30, 206 (1950). 



"^ K. Linderstr0m-Lang, Proc. 6th Intern. Congr. Exptl. Cytol., Stockholm p. 1 (1947). 

 '«« F. Haurowitz, Quart. Rev. Biol. 24, 93 (1949). 

 18' F. Haurowitz, Biol. Revs. 27, 247 (1952). 



'82 H. Chantrenne, 2nd Symposium Soc. Gen. Microbiol, p. 1 (1953). 

 '83 J. Brachet, Exposes ann. biochim. med. 12, 1 (1950). 



