CONTROL BY REPRESSION 37 



can abolish the repiessibihty of the enzymes m this pathway. The 

 "repressible" allele is dominant witli respect to the "non-repressible" 

 one. Maas (1960) has described mntants of E. coli in wliicli argi- 

 nine no longer represses the formation of enzymes in its pathway, 

 and Gorini ( 19601) ) has isolated arginine-repressible mutants from 

 a wild-type strain (B) of £. coli that is non-repressible with respect 

 to enz\ mes of the arginine pathwa\ . Thus, in the arginine system, 

 too, control appears to be exerted by structural as well as regulatory 

 genes, and the repressibilitv character can be studied in genetic- 

 transfer experiments (Gorini, 1960a). Additional complexities of 

 the arginine system are indicated by the finding that relatively high 

 levels of the repressible ornithine transcarbamylase can be produced 

 under the influence of a genetic factor distinct from the one that 

 controls repressibilitv (Gorini, 1960a). Alleles controlling repres- 

 sion have also been explored in the alkaline phosphatase system of 

 E. coli (Echols et at, 1960). 



Horowitz et aJ. ( 1960 ) have studied the induction of tyrosinase 

 formation in N. crassa and concluded that different genes appear to 

 control the structural and the regulatory aspects of the tyrosinase 

 system. Interestingly enough, sulfate acts like a repressor of this 

 system. These authors found that, in a wild-type strain, the enzyme 

 is inducible on a high-sulfate medium but gives the appearance of 

 being constitutive on a low-sulfate medium. This seemingly consti- 

 tutive behavior is interpreted as involving self-induction. Horowitz 

 et ah (1960) also isolated certain mutants which are inducible on 

 either medium. 



It was indicated above that genes considered to control the syn- 

 thesis of active repressors, such as the i or Rtiy genes, may or may 

 not exhibit close linkage with their corresponding structural genes. 

 A different situation obtains in the case of the second regulatory 

 locus, o, of the yS-galactosidase system. Two types of mutations can 

 occur at this locus: (a) dominant mutations (o*") causing consti- 

 tutivity that affect both /S-galactosidase and the permease, provided 

 the mutant allele is in the cis position with respect to the z and ij 

 genes (Jacob and Monod, 1959; Jacob et al., 1960a) and (b) reces- 

 sive mutations (o") mimicking deletions by bringing about loss of 

 the capacity to synthesize both the enzyme and the permease. These 

 results have been interpreted in terms of a genetic factor, the oper- 

 ator, corresponding to the o locus which, being adjacent to the 



