GENETIC CONTROL OF ENZYME STRUCTURE 9 



The last two, suppressor gene action and complementation, will be 

 discussed in a later section of this paper. 



In the case of Neurospora, some CRM proteins do not exhibit any 

 enzvmatic activity in anv of the three reactions which are catalyzed 

 by normal Tsase and which are listed in Fig. 1-1. However, some 

 CRM's still retain the capacity to catalyze parts of the total reaction, 

 for example, reaction 2 or reaction 3 but not reaction 1 (Suskind 

 and Jordan, 1959; De Moss and Bonner, 1959; Yanofsky and Stadler, 

 1958; Yanofsky, 1960). Among the Neurospora CRM's which cat- 

 ah'ze reaction 2, there are three types : ( 1 ) those with no pyridoxal 

 phosphate or serine requirement, (2) those with a pyridoxal phos- 

 phate requirement, and (3) those with a pyridoxal phosphate and 

 a serine requirement (De Moss and Bonner, 1959; Bonner et al., 

 1960 ) . There is also a temperature mutant that forms a CRM which 

 can catalyze all three reactions but is metal-sensitiye (Suskind and 

 Kurek, 1957) . This enzyme has an abnormally high apparent energy 

 of actiyation for tryptophan synthesis (Suskind and Ligon, 1960). 

 Thus, in each of these instances, mutation has resulted in either a 

 total loss of all catalytic activity, in a partial loss of activity, or in 

 the formation of an enzyme which is fully functional only under 

 unique circumstances. In those cases where the CRM's do retain 

 some catalytic activity, additional substrate or cofactor requirements 

 may be evident. 



in the case of the coli mutants, both A-CRM's and B-CRM's are 

 found (Yanofsky, 1960). Hence, one finds mutants which make 

 normal A-protein and B-CRM (or are B-CRM-less), as well as those 

 which form normal B-protein and A-CRM (or are A-CRM-less). It 

 is found that in B mutants with B-CRM, the B-CRM permits full 

 activity of the normal A-protein in reaction 2. In the A mutants 

 which form A-CRM, the A-CRM permits normal B-protein to func- 

 tion maximally in reaction 3. In most cases the affinities of the 

 normal protein or the CRM protein with its associated protein part- 

 ner appear to be the same (Maling and Yanofsky, 1960). In one 

 case, where an altered B with low affinity for A is found, the addition 

 of serine is required for effective combination (Gibson et al, 1961). 



While a number of CRM types have been described and the 

 properties of some have been examined, very little is known at the 

 protein level about the CRM-less mutants mentioned earlier. One 

 CRM-less mutant in Neurospora, strain tdi, does form some com- 



