90 Cell Interactions during Growth and Morphogenesis 



Three kinds of mutants have been isolated: 



Mutant 1 could grow if supplied with ornithine, citrulline, or ar- 

 ginine (i.e., the enzyme that catalyzes reaction 1 is miss- 

 ing)- 



Mutant 2 could grow if supplied with citrulline or arginine, but not 

 if supplied with ornithine. However, ornithine accumu- 

 lated in the cells and seeped out into the medium (i.e., 

 the enzyme-catalyzing reaction 2 was missing ) . 



Mutant 3 could grow only if supplied with arginine, not with orni- 

 thine or citrulline. However, citrulline accumulated in the 

 cells and seeped out into the medium (i.e., the enzyme- 

 catalyzing reaction 3 was missing). 



If, as shown in Fig. 43, the three mutant types are streaked on agar 

 containing a tiny amount of arginine, synergistic growth occurs. Mutant 3 

 grows very sparingly until the arginine in the medium is used up, and then 

 it stops. It continues, however, to generate energy by oxidation and to 

 carry out a wide variety of biochemical reactions, including the synthesis 

 of citrulline, which accumulates and seeps out into the medium. Mutant 2, 

 growing nearby, can convert the citrulline to arginine. Since mutant 3 

 makes a large quantity of citrulline, mutant 2 can grow very well. If the 

 mutant 2 cells do not lie next to mutant 3, they grow very sparingly, but 

 they stiU can metabolize and accumulate ornithine. Since mutant 1 can 



Fig. 43. Synergistic growth 

 by three mutant types of 

 E. coll. 



