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INTRODUCTIO 



The topic "Microsomal Particles and Protein Synthesis" seemed particularly 

 appropriate for the first symposium of the Biophysical Society. The particles 

 owe their recognition to the electron microscope and the ultracentrifuge. X-ray 

 diffraction studies will undoubtedly contribute to the picture of the structure 

 of the particles; radioactive tracers show the kinetics of formation of the par- 

 ticles and their products; radiation experiments can give other evidence about 

 the role of the particles in protein synthesis. Thus many of the special areas 

 of competence of biophysicists are involved in the study of the particles. 



More important, however, is the timeliness of a symposium devoted to a 

 discussion of these ubiquitous granules. For a number of years circumstantial 

 evidence has accumulated which indicates that ribonucleic acid (RNA) is 

 implicated in protein synthesis. More recently it has been recognized that a 

 large part of the RNA occurs in the form of ribonucleoprotein (RNP) par- 

 ticles. Particles of roughly the same size and composition have been isolated 

 from sources differing as widely as rat liver, pea seedlings, and microorganisms. 

 Accordingly there has developed a widespread faith that the particles are an 

 important part of the machinery for protein synthesis. 



It must be admitted, however, that the evidence is entirely circumstantial. A 

 number of arguments would come immediately to the mind of a lawyer de- 

 fending the particles from the charge of protein synthesis. (1) In vivo experi- 

 ments have shown incorporation of radioactive tracers which is initially higher 

 in the microsome fraction than in the soluble proteins, but the kinetic data are 

 not sufficiently complete to prove a precursor-product relationship. For exam- 

 ple, it is possible that steady-state conditions do not prevail; there is no cer- 

 tainty that both components draw on the same pool of amino acids; only the 

 average of the soluble proteins is measured, whereas individual components 

 might behave quite differently. (2) There are many cases both in complete and 

 in cell-free systems where RNAase has been observed to inhibit protein syn- 

 thesis. In many of these the addition of RNA (not RNP) is sufficient to 

 restore the synthetic activity. (3) Cell-free systems showing unequivocal pro- 

 tein synthesis use cells that are only partially disrupted, and the requirement 

 for particles is not demonstrated. In those systems where the particles have 

 been partly purified, the incorporation data are more suggestive of exchange 

 than of true protein synthesis. (4) No mechanism has been suggested which 

 shows how the structure of the particle is compatible with its function as the 

 template for synthesis of long chains. It appears that the particles have not yet 



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