BIOLOGICAL ROLE OF PENTOSE NUCLEIC ACIDS 477 



behaved in many ways like tobacco mosaic virus. Simultaneously, Jeener 

 and Lemoine** discovered a similar (or identical) protein and carried the 

 matter a step further by crystallizing it ; this crystallizable protein behaves 

 in the same way as does Markham and Smith's- material, i.e., it is immu- 

 nologically identical with the virus, it is noninfective, and it is free of PNA. 

 Whether these abnormal proteins present in virus-infected plants are 

 virus precursors, intermediary stages in virus production, or by-products 

 of the virus is not known. It has recently been suggested by Jeener and 

 Lavand'homme^ that the crystallizable antigen, devoid of PNA, occurring 

 in plants into which tobacco mosaic virus has been injected, represents an 

 intermediary product in virus development which accumulates when the 

 leaves no longer contain enough PNA to make the complete virus. In any 

 case, there is no doubt that proteins serologically very closely related to 

 the plant viruses are noninfective when free of PNA. 



b. Interference with Virus Multiplication by Chemical Analogues of Pyrimi- 

 dines and Purines 



If PNA is really essential for the synthesis of plant viruses, one could 

 expect an inhibition of virus multiplication on the addition of substances 

 which interfere with its synthesis : that this is the case has been shown con- 

 clusively by Commoner and Mercer,*'^ who obtained complete inhibition 

 of synthesis of tobacco mosaic virus by thiouracil at a concentration of 4.3 

 X 10~^ M. This inhibition was partially reversed when uracil was added in 

 concentrations of the same order of magnitude. 



These findings of Commoner and Mercer*'^ have been confirmed by 

 Jeener and Rosseels,'" who recently obtained in addition some quite un- 

 expected results. They found that the inhibition of virus synthesis is greater 

 the smaller the amount of virus present in the leaves to which thiouracil is 

 added. This observation cannot be explained on the basis of a competition 

 between thiouracil in some enzymic reaction during the synthesis of PNA, 

 but rather tends to indicate that thiouracil can be incorporated into the 

 virus PNA and that this incorporation hinders the further multiplication 

 of the modified particles. This interpretation of the facts has been amply 

 confirmed by experiments'" in which S'*-labeled thiouracil was added to 

 leaves infected two days earlier; the concentration of thiouracil was such 

 that the speed of virus multiplication was reduced by 50%. When the virus 



^ R. Jeener and P. Lemoine, Arch, intern, physiol. 9, 547 (1952). 

 * R. Jeener and P. Lemoine, Nature 171, 935 (1953). 

 ' R. Jeener and C. Lavand'homme, Arch, intern, physiol. 61, 427 (1953). 

 8 B. Commoner and J. Mercer, Nature 168, 113 (1951). 



^ B. Commoner and J. Mercer, Arch. Biochem. and Biophys. 35, 278 (1952). 

 "* R. Jeener and J. Rosseels, Biochim. et Biophys. Acta 11, 438 (1953). 



