146 THE BIOSYNTHESIS OF PROTEINS 



When the experimental facts led to the conclusion that an extra- 

 chromosomal factor was involved, that this factor was perpetuated through 

 generations, that it could eventually be lost irreversibly, it was perfectly 

 normal to visualize this factor as a kind of free lance gene. It was indeed 

 commonly referred to as plasmagene, self-dividing particle, self-duplicating 

 particle, extranuclear autoreproducing particulate factor, etc. ... In this 

 perspective it was assumed that acridines, high temperature, tetrazolium 

 salts, etc., interfered with the multiplication of this particle 'endowed with 

 genetic continuity'. The nuclear gene was regarded as controlling the 

 multiplication of the self-reproducing cytoplasmic particle or its function. 

 This was very satisfactory especially in view of the fact that several other 

 biological phenomena appeared to fit into a very similar picture (see Sym- 

 posium sur les Unites biologiques douees de continuite genetique, 1948). 



In the present perspective, however, this interpretation looks less con- 

 vincing than it was only a few years ago. The function of the nuclear gene 

 is better understood at present: it is known that the gene controls the 

 sequence of amino acids in the polypeptide chains (see Chapter I). As a 

 result, questions about the nature and function of the extrachromosomal 

 factor necessary for the synthesis of respiratory enzymes can now be asked 

 in a different manner. One wonders especially whether the object contains 

 an extra piece of information about the arrangement of amino acids in cyto- 

 chrome-a, h and cytochrome oxidase, or whether it is involved in some 

 regulation process. 



The irreversibility of the non-chromosomal mutation, the existence 

 of two types of cytoplasmic respiratory deficient mutants in yeast, the 

 fact that the induction of the mutation is subordinated to cell multi- 

 plication, no doubt suggest that it is due to a change in a particulate object 

 which carries with itself some features necessary to its own perpetuation. 

 But these facts in no way imply that the extrachromosomal factor carries 

 any piece of specific structural information required for its own duplication. 

 There is no positive evidence either that the extrachromosomal factor 

 carries any piece of structural information for the synthesis of the respira- 

 tory enzymes. 



It has been mentioned that the progeny of certain white Euglenae which 

 form after a short streptomycin treatment can recover the capacity of 

 making chloroplasts, although they had no visible chloroplast left. In the 

 same way, some respiratory deficient yeasts in the unstable state which 

 follows a moderate acriflavin treatment can give rise to normal respiring 

 cells. When they are in this unstable state, it is quite certain that yeasts or 

 Euglenae have not lost any part of the structural information required for 

 the synthesis of the respiratory enzymes or of the chloroplastic constitu- 

 ents; it is only the expression of this information that was blocked for 

 several generations (De Deken-Grenson, 1960). 



