GENETIC CONTROL 21 



In certain cases, the suppressor mutation was shown to open a pathway 

 of biosynthesis alternative to the one which was blocked in the auxotroph 

 (Lein and Lein, 1952), e.g. by relieving an inhibition (Strauss and Pierog, 

 1954; Howarth, 1958). 



But all suppressors do not act in this way. Auxotroph behaviour in 

 mutants of the td locus (responsible for tryptophan synthetase) in Netiro- 

 spora can be 'suppressed' by mutations occurring at a great distance from the 

 td locus. With a mutant strain which produces a temperature sensitive 

 enzyme, it was observed that the 'suppressed' mutant still displayed a 

 temperature sensitivity which does not exist in the wild strain (Bonner, 

 1946; Yanofsky, 1956; Suskind, 1957). This suggested that the td locus 

 still controlled the structure of the enzyme and that the suppressor prob- 

 ably affected the operation of the abnormal protein in some indirect way. 



Again, for a tryptophan synthetase mutant of Neurospora, such a mechan- 

 ism has been partly cleared up recently (Suskind and Kurek, 1959). The 

 mutant considered contained an enzyme protein which was much more 

 sensitive to inhibition by zinc ions than the normal enzyme. In a 'sup- 

 pressed' mutant, in which wild type behaviour was more or less restored, 

 the enzyme produced was actually the zinc-sensitive enzyme, but the sup- 

 pressor gene in some way controlled the adjustment of zinc ions concen- 

 tration in the mycelium. As a result, the abnormal protein could fulfil an 

 almost normal enzymatic function in the 'suppressed' mutant, whereas it 

 could not (due to zinc inhibition) in the non-suppressed auxotroph. 



In all the cases considered above, suppressors affect the operation of 

 enzymes or of enzyme systems, and their action is irrelevant to the control 

 of the enzyme structure. These facts do not conflict with the idea that the 

 information relative to the primary structure of an enzyme is located within 

 one restricted piece of genetic material. 



There are cases where the difficulties raised by suppression cannot be 

 completely dismissed at present. For instance, a strain of Escherichia coli 

 which lacks tryptophan synthetase and does not produce any corresponding 

 cross reacting material as judged by the usual immunological criteria, can 

 recover the capacity of making a protein having the immunological and 

 enzymic properties of the normal enzyme, as the result of a 'suppressor' 

 mutation in another region of the genome (Yanofsky, 1958; Yanofsky and 

 Crawford, 1959). 



In another mutant, which does produce an enzymically inactive cross 

 reacting material, a suppressor mutation caused the production of a new 

 enzymically active protein, in addition to the altered one. It was shown, 

 besides, that the locus of the suppressor mutation in itself does not carry 

 the information for making the normal protein. 



These facts indicate that suppressor mutations can indeed change some- 

 how the mode of expression of a gene. This could possibly mean that the 



