REGULATION 141 



the induced synthesis of proteins. Enzymic adaptation in animals — at 

 least for the presently analysed cases — may be irrelevant to the control of 

 enzyme synthesis. 



B. NON-MENDELIAN HEREDITARY FACTORS IN 

 ENZYME SYNTHESIS 



According to the presently accepted theory of enzyme synthesis, the 

 nuclear gene contains information relative to the arrangement of the amino 

 acids in the polypeptides. The information is transferred to some cyto- 

 plasmic constituent, most probably a specific RNA, and the amino acids 

 condense in the genetically determined sequence on the surface of the 

 ribosome. The resulting polypeptide is released and it folds in the correct 

 way, thus giving rise to the active enzyme. 



At some point along this sequence of events, controlling agents intervene, 

 either to trigger or to inhibit the formation of individual proteins or of 

 restricted groups of proteins. 



A few important experimental facts are not clearly integrated into this 

 scheme. For instance, the ability of yeast and moulds to make several 

 enzymes of aerobic metabolism is known to be carried over from parent to 

 progeny in a manner which does not obey mendelian laws of heredity. Let 

 us summarize first some of the observations made by Ephrussi and his 

 group on yeast. 



When a culture of Saccharomyces cerevisiae is plated on a solid medium 

 containing glucose, most of the colonies which develop are of nearly 

 identical size; however, a few colonies of distinctly smaller diameter are 

 always observed. This is not due to contamination or to heterogeneity of 

 the yeast strain used, for the same result is obtained when the culture is 

 initiated with one single cell isolated from a large colony. Such cells con- 

 stantly produce in their progeny a majority of cells identical to themselves, 

 and a few cells of a difierent type, recognizable by the small size of the 

 colonies they form. To the contrary, cells from small colonies give rise to 

 small colonies exclusively; they never recover the capacity of forming large 

 colonies (Ephrussi, 1949, 1953). The change from large colony to small 

 colony type is hereditary, and it is irreversible as far as the present data 

 indicate; the difference in size of the colonies is due to a difference in rate 

 of multiplication of the cells. Actually, this difference is observed only when 

 the cells are grown in the presence of air; in a nitrogen atmosphere, normal 

 yeast does not grow faster than the mutant. It was indeed clearly established 

 that the small colonies mutant is deprived of a functional respiratory 

 system, and that it lacks several important enzymes of the respiratory 

 chain, for instance C)^ochrome oxidase, cytochrome-a and cytochrome-6 



