80 



CONTROL MECHANISMS IN CELLULAR PROCESSES 



thus be formed instead of acidic products during the fermentation 

 of gkicose has been interpreted as a selective advantage in this 



organism. 



Juni ( 1952 ) demonstrated quite clearlv that this acetolactate- 

 forming system and the decarboxylase vv^ere absent from E. coli—an 

 observation that was to cast doubt on the possibility that acetolac- 

 tate could be an intermediate in valine biosynthesis in E. coli. YLow- 

 ever, this discrepancy was clarified when it was observed that in 

 E. coli the acetolactate-forming system had a pH optimum near 8 



TABLE 3-5 

 The Biological Functions of Acelolactate Formation in A. aerogenes 



1. As an intermediate in valine biosynthesis 



O O 



CH.^C— COO- Mg++ CH.C 



H DPT "j 



CH. 



CHCH— COO- 



CH3 NH3+ 



Valine 



O 



// 



CH3C 



CH3CH 



OH 



Acetoin 



(Umbarger and Brown, 1958b). There was no decarboxylase. In 

 addition, there were several other features of the E. coli system 

 which differentiated it from the A. aerogenes system of Juni. The 

 most striking of these was the sensitivity of the E. coli system to 

 valine, the end product of the biosynthetic sequence. 



Thus, two distinct physiological functions are known for the con- 

 version of pyruvate to acetolactate. These are illustrated in Table 

 3-5. One is distinctly concerned with glucose catabolism; the other 

 is a biosynthetic function. Assuming that A. aerogenes formed its 

 valine via acetolactate, it would exhibit both of these functions. In 

 view of the fact that E. coli had two enzymes for threonine deamina- 

 tion, it was of interest to determine whether A. aerogenes performed 



