RNA AND CONTROL OF CELLULAR PROCESSES 111 



should be controlled 1)\ the functional state and quantity of enzyme 

 formin<r sites. Aecordinti to our model, this means the kind and 

 quantity of ribonueleoprotein particles. The quantit\' of enzvme 

 forming sites, i.e., quantity of specific RNA, is directlv dependent 

 on gene activity. Similarly, the time at which a certain protein ap- 

 pears in the cell depends on the presence of protein-forming sites, 

 and the production of these, on the activity of genes. In this chap- 

 ter I would like to discuss whetlier and how the genes control the 

 production and the quantit\' of protein-forming sites of the cell. 



The rate of RNA production by the gene will depend on a spe- 

 cific property of the gene, on the availability of precursors, and on 

 environmental factors. The availability of a precursor may become 

 a rate-limiting factor for different RNA molecules if these molecules 

 are built of different amounts of this precursor. If for example one 

 of the four nucleotides is in limited supply, then it is possible that 

 the RNA molecule carrying more of that nucleotide would be pro- 

 duced at a lower rate than other RNA molecules. However, in all 

 cases where the base composition of RNA was studied ( Belozersky 

 and Spirin, 1960), it was found that the four nucleotides occur in 

 approximately equal amounts and that their ratios vary only to a 

 small extent from organism to organism. Also, according to coding 

 theories (Gamow et ah, 1956) the amount of the four nucleotides 

 in nucleic acid cannot vary greatly, variation existing mainly in order 

 along the chain. A limiting supply of one nucleotide can, therefore, 

 not have an important regulatory power in the production of spe- 

 cific RNA molecules. On the other hand, a limiting supply could 

 be of importance if several genes were competing for precursors. 

 Activation of a paiticular gene could then reduce the production of 

 RNA by other less active genes. 



Gene Activation. From the preceding discussion it follows that 

 all genes should produce RNA molecules at approximately similar 

 rates, unless the genes are in different functional states. It would 

 be highly uneconomical for a cell to produce RNA continuously 

 when it is not needed. On the other hand, if there is need for an 

 excessive synthesis of one protein, it would be unreasonable to 

 accelerate total RNA production to supply the needed RNA in suffi- 

 cient quantities. In such a case, the unneeded RNA should be con- 

 tinuously and specilicallv destroNcd to release enough precursors 

 for the predominant RNA. There must be, then, a mechanism which 



