298 CELL HEREDITY 



td-24 su-S has the same metal sensitivity as that of the mutant enzyme 

 from td-24. This example shows that the td-24 mutant contains an 

 altered enzyme in respect to metal sensitivity, but that the enzyme can 

 be activated by: (a) increasing the temperature at which the strain is 

 grown; (/;) purifying the enzyme in vitro; and (c) the occurrence of 

 suppressor mutations. The fact that the enzyme in td-24 su-24 resembles 

 that of td-24 and not of wild type indicates that the suppression in- 

 volves removal of an inhibitor or some other indirect effect, but not a 

 change in the enzyme. 



Suppressors have been found for some of the tryptophane svnthetase 

 mutants of E. coli in which there do seem to be changes in the enzyme 

 itself. For instance, the A-2 mutant does not produce any A-CRM, but 

 the suppressed strain, A-2 su-2, forms an A protein indistinguishable from 

 wild-type A in its catalytic action, but differing in its binding with the 

 B component. Immunological tests revealed little difference between the 

 A protein of the suppressed mutant and wild tvpe. Another suppressed 

 mutant, A-3 su-S, derived from the A-CRM-former, AS, contains an A 

 component with catalytic activities different from the wild-type A. By 

 purification and fractionation it was shown that the suppressed strain 

 contained two separable A components, one like wild-type A and the 

 other like the A-CRM of the original A-3 mutant. In this example, it is 

 clear that the suppressor mutation has resulted in production of active 

 enzyme not previously present. However, the suppressor gene is not 

 simply a duplication, since it acts only in the A-3 strain and not in 

 other A mutants. Perhaps it is a duplication for only a part of the gene. 



The occurrence of in vitro complementation led to the suggestion that 

 the mechanism involves interactions of the enzymes or the enzyme- 

 forming systems, but not of the genes themselves. This view is rein- 

 forced by the observations of suppression just discussed. Further sup- 

 port for this view comes from the studies of nonchromosomal male steril- 

 ity discussed in Chapter 9. As may be recalled, there are at least two 

 such factors of independent origin in maize, and a number of chromo- 

 somal genes which can partially or fully restore fertilitv in the presence 

 of the sterility factors. The genes are factor-specific, in the sense that a 

 particular gene can suppress a particular factor for male sterility and 

 not others. This system is quite analogous to the allele-specific sup- 

 pressors of tryptophane synthetase mutants, except that the male steril- 

 ity determinants are nonchromosomal, while the genes which interact 

 with them to restore fertility are chromosomal. If there is a unitary 

 explanation for all these observations, it most likelv will involve some 

 steps in the process of enzyme formation, or more generally, of protein 

 synthesis. 



