VII. SYNTHESIS OF RNA AND RIBOSOMES 345 



does the composition of their DNA's, the composition of the template 

 RNA might be expected to be relatively constant and, therefore, in some 

 cells at least, unlike the DNA. Furthermore, different parts of the 

 DNA may transmit information (i.e., direct the synthesis of RNA) at 

 different rates. The activity of the gene determining the structure of an 

 inducible enzyme appears to be markedly increased upon induction. In 

 this event the composition of DNA-directed RNA should reflect the 

 composition of the DNA weighted in proportion to the activity of the 

 different parts. 



Alternatively the whole DNA complement might be replicated as 

 RNA, but subsequently those regions not suitable for protein forming 

 templates would be degraded. In some instances, such as hemoglobin 

 synthesis, the enzyme-forming units are quite stable. Templates might 

 then be expected to show a range of stabilities. Finally, for economical 

 operation of the cell, there should be a sufficient quantity of template 

 material to allow all ribosomes to be active. Thus, as an alternative to 

 the messenger theoiy, template material might be predicted to have the 

 following properties: (a) a constant composition from cell to cell re- 

 flecting the average amino acid composition of the proteins, (b) having 

 a range of lifetimes depending on the stability of the particular enzyme 

 forming unit, and (c) a transient or permanent association with the 

 ribosomes. Material of these characteristics is well known. Ribosomal 

 RNA from bacteria has a relatively constant composition. A wide range 

 of stabilities of template materials can be envisaged if some templates 

 can function only during the eosome stage while others can operate after 

 incorporation into ribosomes. 



The value of this model is that it demonstrates clearly that the 

 template material has not yet been identified. The question is still open 

 and cannot be resolved by kinetic studies. At best, the kinetic and com- 

 positional studies identify material having the properties postulated for 

 "the messenger." Even hybrid formation is not diagnostic of template 

 material as it merely indicates a specific relationship to the DNA. Quite 

 a different approach is needed, such as an assay system to detect tem- 

 plate RNA by its effectiveness in synthesizing a definite protein. Niren- 

 berg and Matthaei have made a spectacular beginning in this direction. 

 In their cell-free system polyuridylic acid senses as template for poly- 

 phenylalanine and polycytidylic acid for polyproline (Nirenberg and 

 Matthaei, 1961). 



D. HOMOGENEITY OF RIBOSOMES 



According to the messenger theory ribosomes could be completely 

 homogeneous except for the attached template component. On the other 



