GENETIC CONTROL OF ENZYME STRUCTURE 15 



A number of partial revertants of Neurospora and coli Tsase mu- 

 tants have been studied, and some have been found to differ from 

 both the wild-tN'pe organism and the true revertant. The tryptophan 

 synthetase formed by these strains appears to be specifically altered 

 in terms of its relative enzvmatic and antigenic properties ( Yanof sky, 

 1960; Esser et ah, 1960) . Furthermore, different categories or groups 

 of partial revertants occur, suggesting that "meaningful" alternate 

 nucleotide sequences can exist within a limited genetic region. In 

 addition, perhaps nucleotide changes not confined to the exact site 

 of the original mutation, but located elsewhere in the Tsase gene, 

 can restore a functional protein. Elucidation of the nature of partial 

 reversion is basic to understanding the possible mechanisms of 

 genetic repair and function. 



The last type of gene action we would like to discuss concerns the 

 problem of suppressor gene action. A suppressor mutation can be 

 defined as one which phenotypically reverses the effect of the pri- 

 mary mutation but which is itself located at a different genetic locus. 

 In the case of the Tsase mutants of both Neurospora and coli, sup- 

 pressor genes exert their effect by permitting mutants to form low 

 levels of functional Tsase and so to grow slowly in the absence of 

 exogenous tryptophan (Yanof sky and Bonner, 1955a). However, 

 in most cases the effect is not complete, and growth is stimulated by 

 the addition of tryptophan. Several important questions arise in 

 considering the suppressor gene effect. The first of these is the ques- 

 tion of the allele specificity of suppressor genes; that is, do suppressor 

 genes which affect one td mutant allele affect others as well? Fig. 

 1-5, compiled from extensive genetic analyses, shows quite clearly 

 that suppressor genes are able to function only in conjunction with 

 specific td alleles (Yanof sky and Bonner, 1955a). It can also be 

 seen that one allele (tdi) is not affected by any suppressor gene 

 ( Yanof sky and Bonner, 1955a ) . Furthermore, repeated occurrences 

 of the suppressor-2 mutation can be isolated, and these all exhibit 

 identical allele specificity ( Yanofsky and Bonner, 1955b ) . Most im- 

 portant, these recurrent suppressor-2 genes, on allelism tests, proved 

 to be non-allelic, indicating that a number of genetic sites are capa- 

 ble of reversing the effects of the tdi* mutation. Similar allele speci- 

 ficity is also found in coli suppressor studies (Yanofsky, 1960). 



A second question bears on the requirements for genetic suppres- 

 sion. Here there appears to be a difference between the Neurospora 

 and coli systems. In Neurospora, CRM-formation is required for 



