GENETIC CONTROL OF ENZYME STRUCTURE 17 



When the suppressed mutants are examined for antigenic as well as 

 enzymatic activity, it is found that exceedingly large quantities of 

 CRM are formed even in the presence of suppressor genes ( Suskind 

 et al, 1955). This would suggest either that the primary mutation 

 is still partially or fullv expressed or that the suppressed mutant 

 enzyme has a very low turnover number. Hence, the question arises 

 as to the nature of the suppressed mutant enzyme. Is this normal 

 protein and has the suppressor gene provided information for the 

 repair of the primarv structure of the molecule? Or might not the 

 suppressor gene be primarily concerned with control of the function 

 of a still altered mutant enzyme? 



In Neurospora, a temperature-sensitive mutant, strain td24, was 

 isolated which forms little or no active enzvme at 25° C and which 

 requires trvptophan at tliis temperature (Yanofsky and Bonner, 

 1955a). Elevation of the growth temperature to 30°— 33° results in 

 slew growth of the organism on minimal medium. One would like 

 to know whether temperature had affected the formation or the 

 function of Tsase in this instance. 



On fractionation of crude mutant extracts which initially had little 

 or very low enzymatic activity but considerable quantities of CRM, 

 it was found that 30-50 per cent of the wild-type enzyme level could 

 be demonstrated (Suskind and Kurek, 1957). Further study re- 

 vealed that this fractionated mutant enzyme has a number of prop- 

 erties which clearlv distinguish it from the wild-type enzyme. These 

 include its sensitivity to low concentrations of zinc, the release of 

 metal inhibition by fractionation and by EDTA, sensitivity to heat 

 and to dialysis, an extremely high apparent energy of activation for 

 tryptophan svnthesis, and immunogenic and antigenic differences 

 ( Suskind and Ligon, 1960 ) . There appears to be a good correlation 

 between the properties of the mutationally altered enzyme in vitro 

 and in the in vivo conditions which permit tryptophan synthesis and 

 subsequent growth. In this situation, therefore, environmental con- 

 trol permits the functioning of a genetically altered Tsase. Only two 

 other means of "activating" the system in vivo are known. One of 

 these is by complementation and the other by suppressor gene action 

 (Lacy, 1959; Yanofsky and Bonner, 1955a). 



To determine whether the enzyme formed in the suppressed 

 mutant is of the normal or the mutant type, the properties of wild- 

 tvpe, td24, and td24Sui.-4Tsases were compared. The results of these 

 experiments are summarized in Table 1-2 (Suskind and Kurek, 



