Gene Action and Operons 



423 



teristics.) The third cistron, /+, specifies the 

 synthesis of a repressor substance which pre- 

 vents j+ and z+ from producing the permease 

 and galactosidase, respectively. When the 

 substrate upon which these enzymes act is 

 present, however, the repressor substance 

 made by /+ is inactivated, so that the forma- 

 tion of these enzymes becomes possible. 

 Accordingly, E. coli of genotype 7+2+/+ can- 

 not produce galactosidase or permease con- 

 stitutively (in the absence of substrate for 

 these enzymes) but can do so inductively (in 

 the presence of enzyme substrate). This 

 genotype provides us with an example of the 

 genetic basis for the phenomenon of induced 

 enzyme formation. Note, in this connec- 

 tion, that it is often found that the formation 

 of an enzyme is repressed by a high intra- 

 cellular concentration of its product, so that 

 a feed-back system is involved (cf. p. 418 

 and also Figure 46-1). 



The order of these genes relative to them- 

 selves and others is: TL . . . Pro . . . 

 (Lac) y z i . . . Ad . . . Gal. Note that 

 all three Lac cistrons specify unique sub- 

 stances. Because /+ produces a repressor 

 substance, which in the absence of substrate 

 is capable of pleiotropic effects — that is, of 

 phenotypic suppression of both y^ and z+, /+ 

 can be called a regulator gene. What is the 

 precise mechanism by which /+ suppresses 

 the action of two other genes which are them- 

 selves nonallelic, even if they are part of the 

 same biochemical sequence? 



With this question in mind, let us examine 

 the consequences of certain mutations in the 

 Lac region. Mutants capable of synthesiz- 

 ing permease and galactosidase constitu- 

 tively might be due to the genotype y+ z+ /, 

 in which the specific repressor is not pro- 

 duced, and the genes y^ and z+ are able to 

 act under all circumstances. It is possible to 

 construct E. coli which are hybrid for the 

 Lac region, by introducing into F~ cells F 

 particles containing the Lac region as chro- 

 mosomal memory (see p. 369). Thus, one 



can obtain an E. coli whose chromosome 

 has >'+ z / (which by itself would make per- 

 mease and Cz protein constitutively) and 

 whose F-Lac particle is y+ z+ /+ (which by 

 itself would make permease and galactosidase 

 only in the presence of substrate). In the 

 hybrid no products are formed in noninduced 

 bacteria (in the absence of inducing sub- 

 strate), while all three (galactosidase, Cz pro- 

 tein, and permease) are formed in induced 

 bacteria (exposed to enzyme substrate). We 

 must conclude from this that a single /+ gene 

 can manufacture repressor substance which 

 prevents the products of both normal and 

 mutant y and z genes from being formed 

 constitutively but not inductively, whether 

 or not these genes are located on the same 

 chromosome segment. In other Vv'ords, the 

 repressor substance is diffusible and can act 

 at a distance. There is some evidence that the 

 repressor substance is RNA. 



Another mutant is found which also per- 

 mits >'+ and z+ products to form constitu- 

 tively, and is, therefore, presumably a muta- 

 tion of /+, say to allele i"". When an ¥-Lac 

 particle of the presumed genotype >'+ z+ /^ is 

 placed in a cell whose chromosome is >'+ z / 

 (which by itself is known to produce permease 

 and Cz protein constitutively), no Cz pro- 

 tein is formed constitutively in noninduced 

 bacteria. Contrary to the assumption made, 

 /^ must be /+ since it is producing repressor 

 capable of repressing Cz protein formation 

 constitutively. If so, in what respect is the 

 ¥-Lac particle mutant? Let us make the 

 supposition that the ¥-Lac particle is mutant 

 at a locus, o+,.the new allele being o", which 

 permits only the r and z loci in the same chro- 

 mosome or particle to act constitutively re- 

 gardless of which allele of / may be present 

 in the cell. If this is so, then the F-Lac 

 particle is genotypically y+ z+ o*" /+ while the 

 genotype of the chromosome can be written 

 y+ z 0+ /, ignoring gene order for the present. 

 On the new hypothesis, the hybrid ought to 

 produce permease and galactosidase consti- 



