STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 135 



Leupold (1951) have concluded, from an analysis of temperature-sensitive 

 mutants, that a majority of all genes (not only those of isolable mutants) 

 affecting biosyntheses are not involved in more than one metabolic process. 

 Since only about one-quarter of aU mutants (if they are all similar to the 

 temperature-sensitive group) have requirements which cannot be supplied 

 exogenously by relatively simple substances, it has been concluded that 

 protems, for example, do not involve the formation of all the possible inter- 

 mediate 23eptides via genetically controlled, distinguishable enzymatic steps, 

 but are somehow assembled from the low molecular weight building blocks 

 in a very few steps (perhaps one) involving very few catalysts (perhaps a 

 single template). Atwood and Mukai (1953), using a technique permitting the 

 preservation of mutant lethal genes in heterocaryons, consider that only a 

 small percentage of aU mutants have been isolated and analyzed and that in 

 the majority of mutants we are miable to supply the required metabolite 

 whose synthesis has been blocked. Such a result, of course, casts doubt on 

 the validity of Horowitz's conclusion. 



It is possible that a more direct approach to this problem can be made, 

 since properties transformable by DNA are beginning to be known that can 

 be analyzed at the enzymatic level. Thus, Marmur and Hotclikiss (1955) have 

 described the transformabHity in Pneumococcus of the abihty to metabolize 

 mannitol. The transformed cells are able to form the new enzyme, mannitol- 

 6-phosphate dehydrogenase, which catalyzes the conversion to fructose-6- 

 phosphate. Thus, one may now hope to explore the events between the 

 addition of DNA to the bacterium and the induced biosynthesis of the new 

 enzyme, an experimental advance obviously important in analyzing the mode 

 of action of genetic material. As noted earher, the viruses have also provided 

 two perhaps comparable experimental materials, in which the production of 

 new enzymes occurs after incorporation of new genetic units. For the T-even 

 phages, there are the cases of the evocation of the deoxyuridyhc acid hydroxy- 

 methylase by infection of E. coli strain 15t~, as weU as the apparent 

 induction of deoxycytidyhc acid hydroxymethylase by infection. These 

 events are also possibly controlled by the addition of new DNA. The evoca- 

 tion of neuraminidase by influenza virus which contains only RNA is possibly 

 of particular mterest in dissecting the mode of action of this genetic material. 



C. Some Nutritional Phenomena 



As pointed out earlier, organisms unable to make the prosthetic group of 

 key enzymes as a result of genetic blocks are nevertheless able to make the 

 protein portion of the enzyme, a point of some theoretical interest. In a 

 recent study of Burch et at. (1956) on the development of riboflavin deficiency 

 and the effect of reahmentation of the vitamin in rat tissues, it was observed 

 that the concentration of the various apoenzymes in riboflavin-deficient 



