398 MAHLON B. HOAGLAND 



Nevertheless, using the techniques and conditions found to be effective 

 in evolving active mammalian incorporation systems, Lamborg and 

 Zamecnik 214 have obtained active incorporation of C 14 -amino acids into 

 protein which is clearly dependent on ribosomes, soluble enzymes, ATP, 

 GTP, and an amino acid mixture. Thus it is possible to obtain a system 

 from bacteria which has all the properties of the more clearly defined mam- 

 malian systems, and it may be that further study of such systems will 

 clarify the apparent discrepancies. 



It has been stressed earlier that bacteria possess amino acid activating 

 enzymes, transfer RNA, and ribosomes in all respects very similar to mam- 

 malian cells. A question which remains unanswered, therefore, is the extent 

 to which this membrane material contains bound particles. The material 

 certainly contains RNA and it is possible that only those ribonucleoprotein 

 particles physiologically associated with the cell membranes are capable of 

 completing the protein synthetic sequence. One is reminded that in mam- 

 malian cells the particles are frequently associated with the membranous 

 component of the endoplasmic reticulum and the most active incorporation 

 occurs in microsomes which are, in essence, particles with attached mem- 

 branes. There is growing evidence that the membranous component is a 

 saclike storage compartment for protein synthesized in particles and that 

 the removal of the protein from the particle into this compartment is an 

 active, energy-requiring process. Thus, it might be that in bacteria the cell 

 membrane serves in a capacity similar to the membrane of the endoplasmic 

 reticulum. The disruption of the bacterial cell may bring about an irreversi- 

 ble separation of particles from the cell membrane with resultant loss of 

 apparent synthetic ability; the particles do incorporate amino acids but a 

 net increase in protein cannot be observed because the finished protein 

 molecules cannot be removed from the site of synthesis. Be that as it may, 

 we shall expect that further work on the bacterial systems will do much to 

 shed light on the general mechanism. 



III. Theoretical Considerations 



1. The Role of Ribonucleic Acid 



It can no longer be doubted that ribonucleic acid is intimately involved 

 in the protein synthetic mechanism. Although much of our information is 

 derived from systems of limited synthetic capacity, we have seen that pep- 

 tide-bond condensation occurs in ribonucleoprotein particles and that amino 

 acids are brought to these particles in an activated form by virtue of having 

 been first attached to ribonucleic acid molecules. The essential riddle which 

 the newer knowledge poses for us is : why are two distinct kinds of ribonu- 



214 M. Lamborg and P. C. Zamecnik, Biochim. et Biophys. Acta, in press. 



