Gene Action and Operons 



425 



REGULATOR 

 GENE 



OPERON 



.A, 



Operator Structural 

 gene genes 



O . A . B 



1— t 



REPRESSOR 

 SUBSTANCE 



"LTLT 



' 1 I 



NAAA /\/\ 



1 I 



Proteins 



FIGURE 46-3. Relationships be- 

 tween regulator, operator, and struc- Metabolite 

 tural genes. {After F. Jacob and J. removes 

 Monod.) repressor 



verse mutation, show complementation 

 neither with z nor with y mutants, are ex- 

 tremely closely linked to the C mutant, and 

 are located between z and /. These mutants 

 are clearly alleles of o+, which we can 

 call o". 



It should be noted that the operator gene 

 does not seem to produce any unique product 

 which can be detected cytoplasmically. In 

 this respect it may be considered a gene 

 whose primary job is not the specification of 

 a chemical product, such as the amino acid 

 sequence in a polypeptide, but one whose 

 primary effect is to control the function of 

 other genes. Accordingly, operator genes 

 may be called genes for function in contrast 

 to those which specify chemical structures 

 and are, therefore, genes for structure (Fig- 

 ure 46-3). 



It is possible to explain the action of 

 an operator gene on the basis that it shares 

 nucleotides with an adjacent cistron which 

 is controlled by the operator gene. When 

 the operator gene is functional the adjacent 

 gene cannot be read correctly, since some 

 of its nucleotides are unavailable for this 

 usage. On another occasion, the operator 

 gene might not be functional, permitting 



the adjacent gene to act. Moreover, the oper- 

 ator gene could mutate in such a way 

 that it is no longer ever functional, in which 

 case the adjacent genes would always be 

 functional, or in such a way that the opera- 

 tor gene is always functional, in which case 

 the adjacent genes could never be functional.) 

 (See pp. 370-372 for other applications of the 

 hypothesis of nucleotide-sharing.) 



We have seen that an operator gene coordi- 

 nates the expression of adjacent genes. In 

 the present case the genes controlled are 

 related, in that they both affect the biochemi- 

 cal pathway involving lactose utilization. 

 This suggests that there is, at least in some 

 cases, a unit of gene function which is inter- 

 mediate in size between the cistron and the 

 chromosome, and which we may call an 

 operon. Operons, linear groups of genes 

 whose structural activity is coordinated by a 

 functional gene, or operator, may be common 

 in microorganisms. They may also occur in 

 other organisms and may be more frequent 

 than one might at first suspect.^ 



2 The preceding discussion of operons and operator 

 genes is based upon the work of F. Jacob, D. Perrin, 

 C. Sanchez, and J. Monod (1960), and F. Jacob and 

 J. Monod (1961). 



