68 CONTROL MECHANISMS IN CELLULAR PROCESSES 



How striking is this economy of synthesis for one amino acid, 

 isoleucine, was shown l)v isotope competition experiments performed 

 by the Biophysics Group of the Carnegie Institution's Department 

 of Terrestrial Magnetism (Roberts et al., 1955). These workers 

 showed that when E. coli was grown in a minimal medium contain- 

 ing glucose uniformly labeled with carbon-14 and unlabeled isoleu- 

 cine, the isoleucine in the cells contained no more than 5 per cent 

 of the glucose (labeled) carbon atoms. In other words, the cells 

 had utilized the isoleucine in the medium in preference to making 

 their own. In similar experiments, it was shown that for most amino 

 acids there was either partial or complete suppression of endogenous 

 synthesis. For a few amino acids, however, the cells continued syn- 

 thesis of the amino acid to the same extent whether or not it had 

 been available exogenously. Obviouslv, the question arises: By 

 what mechanism did the cells respond to the exogenous amino acid? 



The interest of physiologists employing mutant methodology for 

 the elucidation of biosvnthetic pathways became focused on this 

 question as a result of observations on the kinetics of the accumu- 

 lation of precursors by auxotrophic (nutritionally deficient) mutants. 

 In the early studies, investigators were not at all surprised to find 

 that certain auxotrophic mutants accumulated compounds that could 

 be utilized for growth by mutants blocked earlier in the same path- 

 way. This property of mutants was the l^asis for syntrophism which 

 occurred when mutants blocked at different points in the same path- 

 way were cultivated in the same tube or were streaked side by side 

 on solid media (Davis, 1950). The early investigators were sur- 

 prised, however, by the fact that the accumulation of precursors had 

 a peculiar relationship to growth which is idealized in Fig. 3-1. 



It was noted that if the mutant was grown in a series of flasks 

 containing increasing amounts of the growth factor, the amount of 

 growth obtained after, for example, a 30-hour incubation would be 

 nearly proportional to the amount of growth factor added until 

 some other factor, such as carbon source, became the limiting one. 

 When such cultures were examined for precursor accumulation, how- 

 ever, it was observed that the point of maximal growth was not the 

 point of maximal precursor accumulation. 



Alternatively, if a kinetic experiment was performed using a single 

 level of growth factor, it was observed that accumulation did not 

 begin until just shortly before growth stopped, owing to growth 



