The genes in E. coli that encode the three enzymes for using lactose are structural genes; structural genes specify the primary structures (the amino acid sequences) of protein molecules that act as enzymes or cytoskeletal proteins. The three genes are adjacent to one another on the E. coli chromosome. This arrangement is no coincidence: the genes share a single promoter, and their DNA is transcribed into a single, continuous molecule of mRNA. Because this particular mRNA governs the synthesis of all three lactose-metabolizing enzymes, either all or none of these enzymes are made, depending on whether their common message—their mRNA—is present in the cell.

339

A cluster of genes with a single promoter is called an operon, and the operon that encodes the three lactose-metabolizing enzymes in E. coli is called the lac operon. The lac operon promoter can be very efficient (the maximum rate of mRNA synthesis can be high), but mRNA synthesis can be shut down when the enzymes are not needed. In addition to the promoter, an operon has other regulatory sequences that are not transcribed. A typical operon consists of a promoter, an operator, and two or more structural genes (Figure 16.4). The operator is a short stretch of DNA that lies between the promoter and the structural genes. It can bind very tightly with regulatory proteins that either activate or repress transcription.

There are numerous mechanisms to control the transcription of operons; we will describe three examples: