STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 137 



respective atoms in catalytic centers, e.g., Fe in peroxidase, Cu in pol3q3henol 

 oxidase and ascorbic acid oxidase (Nason et al., 1952). However, a Zn 

 deficiency doubled the ascorbic acid oxidase content of the plant and copper- 

 deficient tomato leaves showed 10- to 30-fold increases in isocitric dehydro- 

 genase per unit weight of protein, as contrasted to normal leaves (Nason, 

 1952). These increases did not arise from effects on inliibitors or activators 

 concentrations but appeared to represent increases of the enzymes involved. 

 Thus, the apparent plasticity could be expressed in determining the relative 

 proportions of cellular enzymes. 



It may be remarked that the causal relations of these phenomena are quite 

 obscure at the present time. However, it is well known that unusual meta- 

 bolites may accumulate in many plant virus diseases and indeed be useful in 

 the diagnosis of such infection. Although the relation of these metabolic 

 disturbances to virus multiphcation is not known, there seems no reason to 

 doubt that such interconnections might be traced. The problem of the origin 

 of compomids appearing in infected plants that are not fomid in comparable 

 amounts m the normal plant is relevant to many problems of normal plant 

 metabohsm. In analyzing the problem of the distribution, function, and 

 synthesis of the alkaloids, for example, it has been suggested that these 

 compounds cannot aU be thought of as useful in plant metabolism, and that 

 the metabohc pathways involved are probably not aU maximally efficient. 

 It seems more reasonable to imagine that random mutation may increase the 

 production of some metabolites and, as a result of the accumulation of these 

 reactive substances, open routes to new and possibly useless end products. It 

 appears possible that this type of metabolic disorder may also exist at the 

 level of protein synthesis in plants, and indeed account for the production of 

 normal and pathological protems in many virus diseases, e.g., masked strain 

 of TMV, as long as the foreign unit is relatively inert and does not deplete 

 metabolites essential to normal synthesis. 



D. TJte Induced Biosynthesis of Enzymes 



1. Ge?ieral Remarks 



From the point of view of the control of enzyme synthesis and mdeed 

 with respect to the origin of enzymes, the phenomenon of the induced 

 biosynthesis of enzymes has been explored and analyzed to a greater 

 extent than any other (Monod, 1947; Monod and Cohn, 1952; Spiegelman, 

 1951; Cohn, 1957). Until several years ago, the study of this phenomenon 

 provided the most complete body of information concerning the 

 mechanism of synthesis of specific proteins in intact cells; most recently, 

 it has been possible to press this analysis to protoplasts and to cell 

 fragments, bypassing barriers presented by cell walls and membranes. It is 



