STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 



121 



Indeed, a recent study of Thompson and Ballon (1956) has revealed that in 

 the rat some proteins, such as coUagen, show a marked stabihty, not being 

 replaced during the lifetime of the animal. Most chemical fractions isolated 

 from this animal contain large amounts of long-lived components. 



The vahdity of the concept of a dynamic state of any protem has been 

 called to question in recent years by the discovery of Monod and his colla- 

 borators that many amino acid-requiring mutants of bacteria could not be 

 induced to make a new protein in the absence of the required amino acid in 

 the medium. Smce the new protein would probably not exceed 1 % of the 

 protein of the cell, this imphed that not even 1 % of the bacterial protein 

 could be degraded and release the required amino acid for the resynthesis of 

 a new protein (Monod et al., 1952). It was then shown (Hogness et al., 1955; 

 Cohn, 1954) that ^-galactosidase, synthesized in nonisotopic medium in 

 heavily labeled bacteria was nom-adioactive and remained in this condition 

 despite subsequent growth in isotope, as in Fig. 22, a and b. Furthermore, 



n 



D 



Radioactive S /\ Nonradioactives 



No inducer Sulfur inducer 



starvation 



Factor increase of total mass: 12 x 

 (a) 



Non radioactive 

 S inducer 



Sulfur 

 starvation 



Raaioactive S 

 No inducer 



Factor increase of total mass: 12 x 

 (b) 



Fig. 22a. The synthesis and stability of nonradioactive )8-galactosidase (Cohn, 1954). 



Diagrammatic representation of experiment in which Escherichia coli were grown m 

 the presence of radioactive S (crosshatched areas) without inducer until lack of S pre- 

 vented further growth (I). Following the S starvation, nonradioactive S plus inducer 

 were added, whereupon growth was immediately resumed. The jS-galactosidase (small, 

 clear rectangle in II) manufactured from nonmarked S within cells with marked proteins 

 were then analyzed for their isotope content. 



Fig. 22b. The stability of /S-galactosidase (Cohn, 1954). 



Diagrammatic representation of experiment in which Escherichia coli were grown in 

 S starvation on nonradioactive S plus inducer (I). The inducer was then removed and 

 radioactive S was added. The radioactive bacterial protems (crosshatched area) increased 

 12 times, but the enzyme (clear rectangle) remained constant (II). 



labeled enzyme produced in isotopic medium remained intact, as in Fig. 23, 

 despite growth of bacteria in unlabeled media in the presence or absence of 

 an inducer of additional enzyme synthesis. The formed proteins of E. coli 

 were static in all conditions of growth and enzyme synthesis, a fact now 

 confirmed by numerous mvestigators (Rickenberg et al., 1953; Rotman and 

 Spiegelman, 1954; Koch and Levy, 1955). It could be estimated that the rate 



