XL VIRUSES AND NUCLEIC ACIDS 111 



2. The Action of Nucleic Acids 



When the assimilase activity of protoplasm is inferior to that of a 

 virus, the virus can multiply in the protoplasm. On the other hand, 

 the superiority in this activity on the virus part should arise from its 

 rigid structure. Hence, if nucleic acids are inserted among protein 

 molecules to make its structure rigid, the virus will become stable 

 enough at least not to be assimilized by other assimilases. 



Even in this respect alone, particles containing nucleic acids are 

 expected to be an assimilase stronger than those containing no acids. 

 Furthermore, the assimilase activity itself will become stronger, if 

 more rigidly and more regularly are held together the protein mole- 

 cules by nucleic acid, since the assimilase action is considered to be 

 raised from the regular array of protein molecules. 



The purified preparation of turnip-yellow mosaic virus has been 

 found by Markham and Smith (50) to consist of two parts, about 

 80 per cent of particles containing nucleic acid and 20 per cent of a 

 nucleic acid-free ones ; this latter apparently non-infective, but share 

 many properties of the infective ones containg nucleic acid. Thus, 

 the both have similar electrophoretic mobility and form mixed crystals, 

 and appear to contain the same antigens and to react fully with virus 

 antisera. X-ray measurements on crystals of the two kinds of parti- 

 cles indicate that those containing nucleic acid are slightly smaller, 

 suggesting that nucleic acid may hold the particles in a tighter mass. 

 The separation of these two fractions is accomplished by ultracentri- 

 fugation, the bottom component being the protein containing nucleic 

 acid. This greater sedimentation rate of the bottom component may 

 result from the presence of nucleic acid, which holds tightly the pro- 

 tein molecules, while in the particles containing no nucleic acid, the 

 combination among protein molecules may be loose, thus being not 

 easily sedimentable. 



The failure in proving the virus activity in this fraction may 

 likewise be attributable tof-, this slackness of the combination ; however, 

 it may be impossible to conclude that this fraction had always no 

 virus activity, since it might not entirely be impossible to demonstrate 

 its infectivity, if a suitable host with a high susceptibility was used 

 under a proper environmental condition. 



According to Heriott (162) a phage preparation which have been 

 rendered nucleic acid free by "osmotical shocking" can still be adsor- 

 bed onto the host cells, prevent their multiplication and even lyse them, 

 although the phage can reveal only less than 1 per cent of its ori- 

 ginal infectivity, a fact which may indicate that nucleic acid is not 



