Lactose Absent

Repressor protein

Absence Lactose

lac operon figure 11-1

lac operon figure 11-1

In the lac operon of E. coli, three structural genes code for the enzymes needed to utilize lactose.When lactose is absent, a repressor protein attaches to the operator. The presence of the repressor protein on the operator blocks the advancement of RNA polymerase.

□ internet cortneci

www.scilinks.org Topic: Gene Expression Keyword: HM60642

□ internet cortneci

www.scilinks.org Topic: Gene Expression Keyword: HM60642

Lactose is a disaccharide that is composed of the two monosaccharides glucose and galactose. When E. coli bacteria are in the presence of lactose, the lactose induces E. coli to produce three enzymes. These three enzymes control metabolism of lactose. The production of these enzymes is regulated by three elements found within the DNA of E. coli:

• Structural Genes Genes that code for polypeptides are called structural genes. The structural genes studied by Jacob and Monod code for enzymes that allow E. coli to metabolize lactose. The three structural genes that code for these three enzymes are located next to each other on the chromosome.

• Promoter Recall that a promoter is a DNA segment that is recognized by the enzyme RNA polymerase. This enzyme then initiates transcription.

• Operator An operator is a DNA segment that serves as a kind of "switch" by controlling the access of RNA polymerase to the promoter. Thus, the operator controls the ability of RNA polymerase to move along the structural genes.

The structural genes, the promoter, and the operator collectively form an operon. An operon (AHP-uhr-AHN) is a series of genes that code for specific products and the regulatory elements that control these genes. Researchers have found that the clustered arrangement of genes that form an operon is a pattern that occurs commonly among bacteria. Jacob and Monod named the operon that they studied the lac operon because its structural genes coded for the enzymes that regulate lactose metabolism. The lac operon, shown in Figure 11-1 above, includes the entire segment of DNA required to produce the enzymes involved in lactose metabolism.

Jacob and Monod found that the genes for the enzymes for lactose utilization were expressed only when lactose was present. How were the bacteria able to shut off these genes when lactose was absent? Their research showed that gene activation in the lac operon depends on whether the operon is "turned off" or "turned on."

Operon "Turned Off

In the absence of lactose, a protein called a repressor attaches to the operator. A repressor protein is a protein that inhibits genes from being expressed. Repressor proteins are coded for by regulator genes, which are located some distance from the operators they affect. The attachment of the repressor protein to the operator physically blocks the advancement of RNA polymerase toward the structural genes and thus inhibits transcription. Figure 11-1 shows how the attachment of the repressor protein to the operator (the "switch") causes the lac operon to "turn off."

Operon "Turned On"

When lactose is present in the E. coli cell, lactose binds to the repressor protein. This binding changes the shape of the repressor protein. The change in shape causes the repressor protein to detach from the operator ("the switch"), as shown in Figure 11-2 below. RNA polymerase is no longer blocked from transcribing the structural genes of the lac operon. The operon—including the three structural genes—is now "turned on," so all three enzymes required for lactose metabolism are produced. Because it activates, or induces, transcription, lactose acts as an inducer. An inducer is a molecule that initiates gene expression.

The lac operon illustrates in simple terms the great advantage of regulating gene expression. Cells of E. coli are able to "turn off" or "turn on" lactose metabolism depending on whether lactose is present. Because lactose acts as an inducer, the lac operon is "turned on" only in the presence of lactose. As a result, lactose induces its own metabolism. When the level of lactose drops, the repressor protein again attaches to the operator, which "turns off" the lac operon. Therefore, the three enzymes used in lactose metabolism are not produced when lactose is not present. By controlling gene expression, E. coli bacteria conserve resources and produce only those proteins that are needed.

figure 11-2

When lactose is present in an E. coli cell, lactose acts as an inducer by binding to the repressor protein. The repressor protein then changes shape and detaches. The detachment of the repressor protein allows the transcription of the three structural genes to proceed, and mRNA is produced.

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