18 



CONTROL MECHANISMS IN CELLULAR PROCESSES 



1959; Siiskind and Ligon, 1960), and they permit several conclu- 

 sions to be drawn. First, the td24Su24Tsase does not resemble wild- 

 tvpe Tsase but does resemble the enzyme from the unsuppressed 

 mutant. Second, td24Su24 still forms large quantities of CRM. Third, 

 td2 and td2Su2, a pair used for control purposes, resemble wild type 

 in those properties examined (Suskind and Ligon, 1960; Yanofsky 

 and Bonner, 1955a ) . Thus, in the case of td24Su24, it appears either 

 that a mutant enzyme is still being formed and that the suppressor 



TABLE 1-2 



A Comparison of the Properties of Tryptophan Synthetase from Wild-Type, 

 Mutant, and Suppressed Mutant Strains of N. crassa 



Strain 



Property Wild Type tdai td2iSu24 



Resistance or 

 sensitivity to 

 zinc (10-=^ M). . . Resistant Sensitive Sensitive 



Stability or 

 liability to 

 overnight dialysis 

 at 4° C Stable Labile Labile 



Stability or 

 liability to 

 55° C for 5 min. Stable Labile Labile 



Apparent energy 

 activation 

 (calories) 9,000-10,000 30,000-50,000 30,000-50,000 



Inimunogenicity and 



antigenicity Standard Abnormal Abnormal 



td2 



tdeSuo 



Resistant 



Stable Stable 



Stable Stable 



. . . 9,000-10,000 



Similar to Similar to 



wild type viM type 



gene in vivo acts to permit this enzyme to function or that there is 

 a very low level of wild-type-like enzyme present which has not been 

 detected. 



In coli, suppressor genes are also found which are specific for 

 certain mutations of the A or the B genes. In one instance there is 

 evidence that a suppressed A mutant, in addition to forming an 

 A-CRM, does form a second A-protein which is effective in catalyz- 

 ing the InGP ^ In + TP reaction. This second A-protein seems to 

 behave like the wild-type A-protein on column chromatography, 

 suggesting that the suppressor gene has initiated the synthesis of 

 some normal Tsase molecules ( Yanofsky, 1960 ) . 



Regardless of the mechanism of action, it is nevertheless indis- 

 putable that a specific suppressor gene can cooperate with a specific 



