RNA Synthesis in Polytomella 187 



kappa particles, etc. If these particles, partly composed of 

 RNA, multiply autocatalytically, the quantity of RNA syn- 

 thesized at any instant would depend on the quantity present, 

 as the study of the evolution of the specific radioactivity of the 

 RNA led us to assume. 



The hypothesis which we present is valid whatever the 

 intimate mechanism of the multiplication of the particles 

 may be. This multiplication could take place by an increase 

 in size of the particle, followed by its fragmentation into two 

 parts, as happens in the mitochondria and the kinetosomes; 

 or it might be that we are dealing with a more complex 

 mechanism, of which the phage multiplication gives a model, 

 which would involve the rupture of the particle into smaller 

 units, the autocatalytic multiplication of the latter, and their 

 final recombination (Luria, 1950). 



The facts observed till now are not in favour of the latter 

 representation. If, after adding the non-labelled phosphate 

 to the culture which has stopped growing, we study the evolu- 

 tion of the specific radioactivity of the RNA of the smaller 

 particles and at the same time that of the larger particles 

 separated from the former by 10 minutes centrifugation at 

 60,000 g, we find that the radioactivity decreases just as 

 rapidly in both fractions from the moment of addition of the 

 excess phosphate. If the smaller particles were the only site 

 of synthesis of the RNA, the specific radioactivity of the RNA 

 would rapidly decrease in this fraction and would start to 

 diminish in the larger ones only after an appreciable delay. 



In conclusion, we think it possible to interpret all the facts 

 known at present by assuming that the Polytomella cells 

 contain a mixture of various categories of specific particles, 

 each of them being characterized by its size, its RNA content, 

 a collection of enzymes, etc. In each of these categories there 

 takes place autoduplication, the rate of which may be different 

 from the rate of cellular division, and may differ from one 

 category of particles to another. The normal ribonucleo- 

 protein particles would thus possess the characteristics of 

 Sonneborn's plasmagenes (Sonneborn, 1949, 1950) but would 



