27-13 THE YEAST CELL 



defined, and which Sonneborn later used with Darlington's meaning 

 to explain the Kappa phenomenon. However, since neither Darling- 

 ton's nor Sonneborn' s arguments have justified any other termino- 

 logy than "virus" for the entities with which they were concerned, 

 Spiegelman's use of the word can be considered unique. Spiegelman 

 redefined plasmagenes as autonomous cytoplasmic entities which 

 arise from the nucleus as replicas of the gene and which are cast 

 off into the cytoplasm from the gene itself. He suggests that these 

 hypothetical units can become established in the cytoplasm, es- 

 pecially in the presence of substrate, and multiply in the cytoplasm 

 independently of the gene from which they arose. Darlington's and 

 Sonneborn' s viruses (plasmagenes) may or may not have originated 

 from nuclear genes in some earlier evolutionary period, but they 

 do not arise from the nucleus at the present time. Spiegelman's 

 plasmagenes are being continually produced by nuclear genes, al- 

 though they readily attain autonomy in the presence of substrate 

 and may, according to his hypothesis, persist in the cytoplasm 

 even in the absence of substrate. His hypothesis has its origin in 

 the fact that although a nuclear gene is required to enable a yeast 

 to ferment a specific carbohydrate, once fermentation of the carbo- 

 hydrate begins, it proceeds as if it were simply a reaction between 

 the carbohydrate and the enzyme, occurring presumablyin the 

 cytoplasm. Spiegelman supposes that the adaptation curves indi- 

 cate that the reaction (which is initiated by a nuclear gene) proceeds 

 as an interaction between the substrate and gene repi t ca^which 

 the gene has sent into the cytoplasm. I have developed a different 

 view based on the assumption that the synthesis of a gene -product 

 which confers specificity to a specific fermentative enzyme occurs 

 on the chromosome rather than in the cytoplasm. The gene -pro- 

 duct eventually passes into the cytoplasm and makes the enzyme 

 effective. This hypothesis otjes not require that gene replicas 

 diffuse into the cytoplasm where they exist as autonomous entities. 

 On this view^ the reaction which initiates the process of adapta- 

 tion, as well as the continued synthesis of the key substance, 

 occurs on the chromosome. The substance which is produced by 

 the chromosome and confers specificity is, by hjrpothesis, incapa- 

 ble of autonomy. 



Spiegelman's version of the plasmagene hypothesis takes its 

 origin from the fact that adaptive enzyme formation follows a 

 sigmoid curve characteristic of an autocatalytic reaction. He 

 reasons that this supports the view that the enzyme / tself must 

 be autonomous. Monod argues against this interpretation. Monod 

 says that the formation of an adaptive enzyme like galactozymase 

 which is involved in the energy processes of the cell might appear 

 to progress by an autocatalytic process simply because the energy 

 which it releases from galactose is the limiting factor in the syn- 



