The Operon: A Group of Genes Whose Expression is Coordinated by an Operator

The analysis of different bacterial systems leads to the conclusion that in the synthesis of certain proteins (enzymatic or viral) a double genetic determinism intervenes involving two genes with distinct functions: one (the gene for structure) responsible for the structure of the molecule, and the other (the regulator gene) governing the expression of the former through the intermediary action of a repressor (I). The regulator genes which have so far been identified show the remarkable property of exercising a pleiotTophic coordinated effect, each governing the expression of several genes for structure, closely linked together, and Corresponding to protein enzymes belonging to the same !nochemical sequence. To explain this effect, it seems necessary to invoke a new genetic entity, called "operator," which would be: (a) adjacent to a group of genes and would control their activity; and (b) would be sensitive to the repressor produced by a particular regulator gene (I). In the presence of the repressor, the expression of the group of genes would be inhibited through the intermediation of the operator. This hypothesis leads to some distinctive predictions concerning the mutations which could affect the structure of the operator. In effect: