Cell Interactions during Growth and Morphogenesis 89 



the parental type, had to be supplied with various compounds in order 

 to grow because they could not synthesize the compounds; some examples 

 are histidineless mutants ( which cannot synthesize histidine and will not 

 grow unless supplied with it) and Thiamineless, Riboflavinless, and 

 Tryptophaneless mutants, and many others. These are called biochemi- 

 cally deficient mutants. 



In all cases, the mutant lacks a single enzyme necessary for the 

 synthesis of a particular cell constituent. Amino acids, vitamins, etc., are 

 generally synthesized in stepwise fashion from a pool of common, simple 

 substances in a manner shown symboHcally below: 



gene 1 gene 2 gene 3 gene 4 gene 5 



I 1 1 1 1 



<— I C<| CO ^ ic 



O ^ <D O V 



I I I I i. 



pool > A > B > C > D > cell constituent 



The absence of enzyme 2 would eliminate the reaction A > B and 



thus prevent the production of intermediates B, C, D, and the final prod- 

 uct. If gene 2 in the nucleus mutated to a non-functional form (i.e., one 

 unable to direct the synthesis of enzyme 2), the mutant cell would be 

 unable to synthesize the final product and would grow only when it was 

 supplied in the culture medium. Two predictions can be made for such 

 mutants and they turn out to be true. These predictions are: 



1. That the mutant lacking enzyme 2 could still grow if supplied 

 with the final product, or with intermediates B, C, or D, since the cell can 

 convert these into the final product. 



2. That intermediate A would accumulate in the mutant cell, since 

 the reaction A > B is blocked. Because these intermediates are gen- 

 erally of low molecular weight and are readily diffusible, they not only 

 accumulate inside the mutant cell but usually seep out of the cell into 

 the surrounding medium. 



Synergistic growth of deficient mutants occurs because of this os- 

 motic property. As an example, we shall consider biochemically deficient 

 mutants of the bacterium Escherichia coli, which are all arginineless (un- 

 able to synthesize the essential amino acid, arginine). Arginine is nor- 

 mally synthesized via the following pathway: 



pool of common 

 substances 



2 3 

 ornithine > citrulline * arginine 



