STUDIES ON THE MECHANISM OF 

 PROTEIN SYNTHESIS* 



P. C. Zamecnik, Elizabeth B. Keller, M. B. Hoagland, 



J. W. LiTTLEFIELD AND R. B. LOFTFIELD 



Medical Laboratories oj the Collis P. Huntington Memorial Hospital of Harvard 

 University, Massachusetts General Hospital, Boston 



The investigation of protein synthesis received a great 

 stimulus ten years ago, when the ready availabihty of radio- 

 active isotopes made it possible to pursue in greater detail the 

 line of research initiated by the Schoenheimer school (Schoen- 

 heimer, 1942). After a few fruitful years, however, this 

 approach reached a plateau. During the past two or three 

 years there has again occurred great progress in this field, 

 due to the application of other techniques — centrifugal, 

 microbiological, and cytochemical — to the problem. Two good 

 reviews (Tarver, 1954; Borsook, 1956) have appeared recently, 

 covering early as well as current work in the field. 



Brachet (1941) and Caspersson (1941) first called attention 

 to the high concentration of ribonucleic acid in the cytoplasm 

 of cells of rapidly growing tissues, and in cells of tissues 

 engaged in the secretion of proteins. The work of Palade 

 (1955) and of Sjostrand and Hanzon (1954) has utihzed the 

 high resolving power of the electron microscope to call 

 attention to details of structure of the fine reticular network 

 found in the cytoplasm of many cells. Two principal con- 

 stituents of this cytoplasmic network are (1) the double- 

 walled membranes, and (2) the small ^ense granules visualized 

 in high concentration both on the exterior walls of the tubules, 

 and also as unattached particles. These structures have been 

 particularly evident in secretory cells such as those of the 

 pancreas. 



* Supported by grants in aid from the U.S. Atomic Energy Commission and 

 the American Cancer Society. This is a pubHcation of the Harvard Cancer 

 Commission. 



RAD. 161 7 



