48 



CONTROL MECHANISMS IN CELLULAR PROCESSES 



1960) on the localization of newly formed proteins and ribonucleic 

 acid in hyphae of N. crassa. The ingenious combination of centrifu- 

 gation and autoradiography techniques used is discussed in Chapter 

 4 by Dr. Zalokar. Further evidence in favor of the view that protein 

 synthesis can occur without simultaneous ribonucleic acid formation 

 is furnished by enucleation experiments with certain protozoa. Re- 



+ URACIL 



NO ARG OR URACIL 

 A- 



ARG + URACIL 

 ARG NO URACILy 



1 1 



30 60 



TIME AT 37°C (MIN) 



90 



Fig. 2-7. Synthesis of ornithine transcarbamylase (OTC) under conditions of 

 uracil starvation. Cells of a uracil-requiring mutant strain of Escherichia coli 

 were grown on a giycerol-salts medium, supplemented with uracil (20 /xg/ml) 

 and arginine (200 /ig/ml), and were starved in this medium modified by the 

 omission of uracil, for 75 minutes at 37° C. The cells were then suspended in 

 unsupplemented medium and divided into 4 portions: the first portion received 

 uracil (solid triangles); the second, arginine (solid circles); the third, uracil and 

 arginine (plain circles); and the fourth, no addition (plain triangles). Incubation 

 was carried out at 37° C with shaking, and samples were removed, as indicated, 

 for enzyme and protein determinations. See Rogers and Novell! (1959) for the 

 procedures and the enzyme unit used. {Courtesy, P. Rogers and G. D. Novelli.) 



