362 MAHLON B. HOAGLAND 



required the higher temperatures to disrupt. It is suggested that the weaker 

 third of the hydrogen bonding capacity of particle RNA may be reserved 

 for interaction with the structural protein. The authors suggest that the 

 particle RNA may consist of areas (totaling about 25 % of the total chain 

 length) of helical continuity alternating with nonhelical lengths, the latter 

 being available for interaction with protein, or other molecules. The X-ray 

 diffraction photographs of RNA obtained by Rich and Watson 57 are con- 

 sistent with this kind of picture. The possible significance of such a struc- 

 tural arrangement of RNA in terms of protein synthetic mechanisms will 

 be discussed more fully below. 



We may thus summarize this consideration of ribosomes as physicochem- 

 ical entities by saying that preliminary studies indicate that they are ubiq- 

 uitous spherical cellular constituents variously attached to other cellular 

 membrane components or free in the cytoplasm, which undergo physiological 

 changes in molecular size. They consist of equal parts of relatively highly 

 polymerized RNA and protein. Both the RNA and protein would appear 

 to be made up of subunits of undetermined size, most likely bound to each 

 other by MgT 1 ' linkages. The protean moiety would appear to function as 

 a structural matrix for the UNA. 



b. Enzymic Properties of Ribosomes 



Analysis of the enzyme content of particles is in an early stage and very 

 little information is available. A number of considerations make interpre- 

 tation of results difficult, first, since particles are associated, particularly 

 in mammalian systems, with much other protein material which is known 

 to have enzymic activity, it is hazardous to assume that any enzymic ac- 

 tivity is intrinsic to the ribonucleoprotein particle. Secondly, it is reason- 

 able to expect that enzymes intimately involved in protein synthesis might 

 be only loosely bound to the surface of the particles and during rigorous 

 attempts to purify the particles they might be lost. This might, for example, 

 be the case with the amino acid activating enzymes. Finally, since particles 

 arc the site of protein synthesis, they are in the process of making enzymes. 

 Indeed, Siekevitz and Palade 40 have reported preliminarily on the finding 

 of synthesized chymotrypsinogen in pancreas particles. A distinction be- 

 tween an intrinsic enzyme necessary to the synthetic machinery and a 

 synthesized enzyme product might be difficult. This is particularly so when 

 one considers that only a small amount of enzyme protein would be neces- 

 sary to give measurable activity. With these considerations in mind it be- 

 comes hazardous to interpret any of the data thus far available. 



Glucose-6-phosphatase, nucleotide-cytochrome c reductase, cytochromes 

 m and b 5 , and enzymes involved in fatty acid and steroid metabolism, 



» A. Rich and J. 1). Watson, Proc. Natl. Acad. Sci. 40, 759 (1954). 



