74 



CONTROL MECHANISMS IN CELLULAR PROCESSES 



pathway. The other kind of control (repression) is shown in the 

 diagram as feedback Loop I in which P or a derivative of P inter- 

 feres with the formation of enzyme a. Since the analogy between 

 feedback in a biosynthetic sequence and that in a Williamson ampli- 

 fier is not so immediately obvious to most liiologists, it may be more 

 profitable to compare the biological system to more mechanical ex- 

 amples of feedback control such as the governor of a steam engine ^ 

 wliich decreases the steam supply at excessive speed and increases it 

 at low speeds. 



Cell Medium 



Glucose 



Cell 

 Constituents 



I 



> > > 



ex 



Enzyme 

 a 



Enzyme 

 Forming 

 System 



Amino Acids 



Fig. 3-3. Diagrammatic representation of feedback loops controlling (/) the 

 synthesis of enzyme a and (//) the activity of enzyme a by the end product of 

 the biosynthetic pathway leading from A to P. For further details, see text. 

 (Reprinted from the Journal of Biological Chemisiry [Umbarger and Brown, 

 1958a] by permission.) 



The fact that the pattern of control which was observed in the 

 isoleucine pathway was discovered at about the same time to be 

 operating in the biosynthetic pathways leading to pyrimidines ( Yates 

 and Pardee, 1956) and to proline (Strecker, 1957) suggested that this 

 pattern might be a general one. In each case the feedback loop 

 involved the end product of the sequence and what presumably was 

 the first enzyme leading specifically to that end product. Therefore, 



-I A more homely, and perhaps self-explanatory, model is seen in the float-controlled 

 valve of a domestic tank-type toilet. The valve (enzyme) is closed (inhibited) com- 

 pletely when the level of water (intracellular pool of product) is sufficiently high. 

 Carrying the analogy further, the factory or assembly line which constructs toilets is 

 like theenzyme-forming system and will shut down or speed production depending on 

 the need for additional units ( enzymes ) . 



