RNA AND CONTROL OF CELLULAR PROCESSES 121 



Tlie feedback control oi hormone activity is known in several 

 other cases involving the hypophysis. Everyone knows today about 

 ACTH (adrenocorticotropic hormone) and cortisone. The over- 

 whelming amount of new literature on this sulDJect prohil^its me 

 from following all the new developments, but I am sure that a case 

 similar to that of thyroid could be presented. A similar relationship 

 also exists between the production of gonadotropic hormones and 

 male and female hormones, or between luteinizing hormones and 

 corpus luteum growth and activity. 



My thesis is that in all these cases, hormones act on the genie 

 sites for hormone production of regulated glands. The hypophysal 

 "tropic" hormones activate the gene sites responsible for the pro- 

 duction of enzymes functioning in hormone synthesis; while the 

 feedback of these hormones acts by repressing the gene sites respon- 

 sible for the production of "tropic" hormones. This mechanism dif- 

 fers from that of induction or repression, since the active molecules 

 are in no way related to the action of the proteins produced. It can 

 then be imagined that activating (or repressing) hormones act on 

 several unrelated genes to influence a set of reactions. The parts 

 of these genes susceptible to activation should then be identical and 

 I would not be surprised if some day it will be found that parts of 

 the protein molecules of enzymes depending on the same activator 

 are identical. 



The nature of hormone action on cell metabolism remains un- 

 known. Many theories try to explain this action by changes in cell 

 permeability, or by activation or inhibition of enzymes. If this is so, 

 hormone action is very remote from the subject of our discussion. 

 Still, I would like to suggest the possibility that in this case also, 

 hormones act directly as gene activators, not for a particular enzvme 

 but for a class of enzymes. To give an example, we could think of 

 sex hormones as key substances to unlock— activate— a set of genes 

 involved in the expression of sexual characters. The antagonistic 

 action of male and female hormones could easily be explained simi- 

 larly to the antagonistic action of enzyme inducers and repressors. 



Gene Action and Cell Differentiation. Until now, I have dis- 

 cussed the changes in protein production, as dependent on RNA 

 and gene activity for the lifetime of cells of one particular type. In 

 a growing, multicellular organism, cells specialize in morphology 

 and function, especially with respect to the kinds of proteins they 



