88 R. MARKHAM 



presence of radioactive orthophosphate (Staehelin, 1958). The rate of labeling 

 increases in the order cytidylic acid, uridylic acid, guanylic acid, and adenylic 

 acid. When the virus nucleic acid is hydrolyzed to give the nucleoside 2'- 

 and 3'-phosphates, the label is uniformly distributed. These effects are, of 



3- 



course, only observable for a short time after giving the V^^O^^ 



M. The Quantity of Nucleic Acid in the Virus 



When the virus is denatured by heat, it appears that the nucleic acid is 

 liberated as one piece. The exact amount of nucleic acid present in the virus 

 particles is a matter of some doubt, but a series of analyses by Black and 

 Knight (1953) on seven strains suggests that the most likely phosphorus 

 content is 0.51 %. This value is not incompatible with most of the other 

 analytical figures which have been reported (the main source of error is in 

 the determination of the actual weight of the virus used for the P estimations); 

 as the average residue weight of the nucleotides is 319, the nucleic acid 

 content of the virus would be 5.27 %, expressed as the free acid. There is 

 some reason to suppose that the nucleic acid may exist as the free acid in 

 this virus, because it is necessary to have salt present in order to separate 

 the two components of the virus. In this case one would have to assume that, 

 as in fish spermatozoa, the phosphate groups were neutralized by the arginine- 

 guanidino groupings, and when the dissociation occurred the salt satisfied 

 the charge requirements. The amount of free nucleic acid liberated would 

 then be increased by the amount of cation which combined with it. Thus if 

 the potassium salt were formed, the percentage would appear to be 5,7 %. 

 With the sodium salt this would be 5.64 %. It is, however, very likely for 

 reasons other than purely esthetic ones that the actual phosphorus percentage 

 should be somewhat greater — about 0.546 %. The X-ray analysis of the 

 virus shows that the nucleic acid is intimately associated with the protein 

 subunits, buried in them, in fact, and it is therefore more or less imperative 

 that the structures of the protein and the nucleic acid should be closely related. 

 It is probable because of this that the number of phosphorus atoms is an 

 integral number per protein subunit, and the most likely integral number 

 is 3 (there is a slight possibility of there being 7 P atoms per 2 subunits, 

 but for various reasons this would seem very unlikely). The quantity of free 

 ribonucleic acid would then be about 5.64 % (this value will vary according 

 to the estimate of the actual protein subunit size). 



Hopkins and Sinsheimer (1955) actually measured the molecular weight 

 of the ribonucleic acid, as liberated in salt solution by heating. The value 

 which they got was 1.7 million compared with the value of 40 million obtained 

 for the whole virus by the same method. The value anticipated would be 

 about 2.5 million per 40 million as the sodium salt, or if one takes the rather 



