VII. SYNTHESIS OF RNA AND RIBOSOMES 341 



which we designate D-RNA and R-RNA to avoid any implication as to 

 their functions. D-RNA indicates an RNA having the same base com- 

 position as the DNA and R-RNA indicates a purine rich RNA. For 

 example, the newly formed RNA might be a mixture of equal propor- 

 tions of D-RNA and R-RNA. The SOS ribosomes having a slightly 

 greater purine excess can then be assumed to contain y^ D-RNA and 

 % R-RNA. The purine excess of the 50S particle is roughly twice that 

 of the SOS ribosome but the kinetics of synthesis indicate that the SOS 

 neosome may be a precursor to the oOS ribosomes. Thus the oOS ribosome 

 may be assumed to contain % D-RNA and % R-RNA. 



Figure SO shows that (with the exception of Proteus vulgaris) the 

 data are consistent with this hypothesis. The accuracy is not sufficient 

 to rule out other possibilities (e.g., that the 50S contains no D-RNA), 

 but this particular one is the most attractive. The reality of the two 

 types is supported by the experiments showing the fractionation of 

 eosomes. There is, however, no direct evidence that the SOS ribosomes 

 contain a D-RNA component. If this hypothesis happens to be correct, 

 then D-RNA and R-RNA would be formed initially in equal quantities 

 but the requirement for ribosome synthesis would be 2 parts of D-RNA 

 for 7 parts R-RNA. 



E. MECHANISMS FOR THE CHANGE IN APPARENT COMPOSITION 



Apparently, more D-RNA is synthesized that could possibly be 

 needed for the production of ribosomes. This interpretation is commonly 

 used as evidence that the D-RNA is subsequently degraded. While this 

 hypothesis may be correct, it poses several problems and it should not 

 be accepted as final until several other alteiTiative interpretations have 

 been excluded. 



The degradation products must be such that they are rapidly re- 

 utilized without mixing with the large pool of mononucleotides of the 

 cell. If this were not the case, TCA precipitable material would be con- 

 verted to TCA soluble material when a short period of uracil incorpora- 

 tion is terminated. The expected degradation products (from ribonuclease 

 action) would be 2',S'-nucleotides which would require phosphorylation 

 to 5'-nucleoside triphosphates before reincorporation. Such a process 

 would be likely to cause mixing with the TCA soluble pool. Perhaps the 

 degradation products are larger and are reutilized without breakdown 

 to mononucleotides by a presently unknown process. Such products 

 could serve as precursors for soluble RNA if the delay of entiy into 

 S-RNA is to be attributed to the pool of D-RNA. 



As these difficulties are not trivial, it is worthwhile examining some 

 of the alternative interpretations. The concept of degradation and 



