37. NUCLEIC ACID AND PROTEIN SYNTHESIS 361 



and cytosine content characteristic of animal tissues. There do not seem 

 to be interspecies differences among animal tissues. A purine-pyrimidine 

 ratio close to unity is also characteristic. Ribosomal RNA from yeast and 

 bacteria is also characterized by near equality of purines and pyrimidines 

 but there is a lack of the high guanine, high cytosine content found in mam- 

 malian ribosomes. 



(4) Macromolecular Organization. It has been pointed out 54 that X-ray 

 powder diagrams of yeast and liver particles, in common with certain vi- 

 ruses, have no resemblance to the diffraction patterns obtained from their 

 isolated RNA or a mixture of their RNA and protein. It would appear 

 that the protein component of particles and viruses can maintain its struc- 

 ture in the absence of RNA, but the "configuration of the RNA in some 

 way closely conforms to that of the protein. Neither the viruses nor the 

 microsomal particles consist of a kind of protein bag inside which lies RNA. 

 This situation may be contrasted with DNA, the in vivo and in vitro struc- 

 tures of which have been shown to be similar. In at least some of the DNA- 

 protein complexes it is the protein which conforms to the structural con- 

 figuration of the nucleic acids." 54 Crick and Watson have called attention 

 to certain similarities between the particles and the spherical viruses in 

 which the protein appears to be arranged in subunits having cubic sym- 

 metry. 55 



Doty and his associates have used the variation in the optical density 

 of polynucleotides as a function of temperature to study what they con- 

 sider may be the intramolecular helical content of particle RNA and whole 

 particles. 56 It appears that when base interactions within and between 

 polynucleotides in solution — presumably hydrogen bonding and other non- 

 covalent associations — are weakened by raising the temperature, the in- 

 crease in optical density of the solution may be used as a direct measure 

 of loss of helical structure. This conclusion is strengthened by concomitant 

 study of changes in optical rotatory properties which would distinguish 

 between lengths of helical continuity along a polymer chain and simple 

 random base pairing. The results of these workers are consistent with the 

 idea that the RNA of these particles and the particles themselves have a 

 substantial content of regular intramolecular hydrogen bonding resulting 

 in helix formation. The whole particles were found to have about two- 

 thirds as much helical content as the RNA isolated from them but this 

 two-thirds was the more strongly interacting type, i.e., the type which 



54 R. E. Franklin, A. Klug, J. T. Finch, and K. C. Holmes, Discussions Faraday Soc. 

 25, 197 (1958). 



55 F. H. C. Crick and J. D. Watson, in "The Nature of Viruses," A Ciba Symposium. 

 Churchill, London, 1956. 



56 P. Doty, H. Boedtker, J. R. Fresco, B. D. Hall, and R. Haselkorn, Ann. N. Y. 

 Acad. Sci. 81,693, 1959. 



