Operon Structure

One significant difference in prokaryotic and eukaryotic gene control lies in the organization of functionally related genes. Many bacterial genes that have related functions are clustered and are under the control of a single promoter. These genes are often transcribed together into a single mRNA. Eukaryotic genes, in contrast, are dispersed, and typically, each is transcribed into a separate mRNA. A group of bacterial structural genes that are transcribed together (along with their promoter and additional sequences that control transcription) is called an operon.

The organization of a typical operon is illustrated in 4 Figure 16.3. At one end of the operon is a set of structural genes, shown in Figure 16.3 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 regulate the transcription of the structural genes of the operon. The regulator gene is not considered part of the operon, although it affects operon function. The regulator gene has its own promoter and is transcribed into a relatively short mRNA, which is translated into a small protein. This regulator protein may bind to a region of DNA 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 16.3).

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