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The organization of a typical operon is illustrated in Figure 12.2. At one end of the operon is a set of structural genes, shown in Figure 12.2 as gene a, gene b, and gene c. These structural genes are transcribed into a single mRNA, which is translated to produce enzymes A, B, and C. These enzymes carry out a series of biochemical reactions that convert precursor molecule X into product Y. The transcription of structural genes a, b, and c is under the control of a promoter, which lies upstream of the first structural gene. RNA polymerase binds to the promoter and then moves downstream, transcribing the structural genes.

A regulator gene helps to control the expression of the structural genes of the operon by increasing or decreasing their transcription. Although it affects operon function, the regulator gene is not considered part of the operon. The regulator gene has its own promoter and is transcribed into a short mRNA, which is translated into a small protein. This regulator protein can bind to a region of the operon called the operator and affect whether transcription can take place. The operator usually overlaps the 3′ end of the promoter and sometimes the 5′ end of the first structural gene (see Figure 12.2).