128 CONTROL MECHANISMS IN CELLULAR PROCESSES 



occurred in 70S particles. The activity of ribosomes may also de- 

 pend on their position in the cell. When ribosomes were separated 

 from other cell constituents by centrifugation, protein synthesis 

 stopped (Zalokar, 1960b, 1961), even in cases where ribosomes 

 were not completelv separated from mitochondria. It appeared that 

 ribosomes, to function normally, should not be separated from endo- 

 plasmic reticulum. 



Many other factors can regulate the synthesis of proteins by RNA. 

 The activity of RNA in the cytoplasm depends on the mass-action 

 of precursors, co-factors and enzymes necessary in protein synthesis. 

 Since all RNA molecules serving as templates differ probably only 

 by the sequence of their nucleotides, it could be expected that they 

 will all be affected equally by the above factors. The availability of 

 precursors may affect synthesis differentially if the proportion of the 

 precursors in the protein varies. If one protein does not contain a 

 particular amino acid, while others do, then this protein will still be 

 synthesized when the amino acid is absent as a precursor. If a pro- 

 tein is made largely of one amino acid, its synthesis rate will be 

 affected more by a limited supply of this amino acid, than would 

 that of other proteins. To my knowledge, no experiments have been 

 made to demonstrate this, although it would be easy to do so with 

 the many available amino acid-less mutants of various microorgan- 

 isms, or with several amino acid analogs. Even if such control 

 existed, it could cope only with a few general classes of proteins and 

 would not be very significant in the exact regulation of cell functions. 



The only mechanisms which could explain an orderly regulation 

 of RNA function would be similar to the ones discussed in RNA 

 production. There is no clear evidence that such mechanisms exist 

 at this level and the arguments presented above were in favor of 

 such controls at the genie level. In the case of some inducible en- 

 zymes, the inducer may act directly on RNA, since new enzyme for- 

 mation starts immediately after induction (Pollock, 1959). This 

 suggests that if DNA had a special mechanism responding to the 

 induction, this mechanism should have been transmitted to RNA 

 during its formation and may function as well in RNA. This would 

 become even more probable if it was found that there is a protein 

 ( or protein-like assembly of amino acids ) involved as an intermedi- 

 ary in the formation of RNA. The examples of inactive RNA men- 

 tioned show that there must be some mechanism to activate RNA 



