Bacteria respond rapidly to changes in their environment. Genes for lactose utilization are activated once E. coli cells sense the presence of lactose or a similar compound

Operon composition/induction - The lactose operon consists of three linked structural genes that encode enzymes of lactose utilization, plus adjacent regulatory sites. The three structural genes--z, y, and a--encode -galactosidase, -galactoside permease (a transport protein), and thiogalactoside transacetylase (an enzyme of still unknown metabolic function), respectively.

In the presence of an inducer, all three enzymes accumulate simultaneously, but to different levels. Lactose itself leads to induction of the lactose operon (also called the lac operon), but the true intracellular inducer is allolactose (Gal(1-->6)Glc), a minor product of -galactosidase action. In the laboratory one usually uses a synthetic inducer such as isopropylthiogalactoside (IPTG), which induces the lactose operon but is not cleaved by -galactosidase. Hence, its concentration does not change during an experiment.

Regulation in the lactose operon - A mutation in a structural gene (z, for example) can inactivate its product (-galactosidase) without affecting control of the other two genes. However, mutations in the regulatory regions (DNA sequences) mapping outside genes z, y, and a can affect expression of all three structural genes.

Operon models - The original Jacob--Monod model for gene regulation, based upon the lactose operon system, is shown schematically in Figure 26.2. A more complete description of the lac operon is shown in Figure 26.17. Transcription of the three structural genes is initiated near an adjacent site, the operator. Transcription yields a single polycistronic messenger RNA (that is, an RNA containing all three genes). The term cistron is used here to indicate a region of a genome that encodes one polypeptide chain.

Lac repressor - The i gene product of the lac operon is a macromolecular repressor which, in the active form binds to the operator, thereby blocking transcription (Figure 26.18a). The repressor also has a binding site for inducer. Binding of IPTG, allolactose, or some other inducer at this site inactivates the repressor by vastly decreasing its affinity for DNA (Figure 26.18b). Inactivating the repressor stimulates transcription of z, y, and a.. Thus, the introduction of lactose or a similar inducer activates synthesis of the gene products involved in its catabolism by removing a barrier to their transcription. This mode of regulation is negative, because the active regulatory element (the repressor) is an inhibitor of transcription. Positive control is also a factor, involving the CRP site shown in Figure 26.17.

Modifications to Model - The following three major modifications to the Jacob-Monod model occurred as the system was subjected to further analysis:

1. The promoter was discovered to be an element distinct from the operator (although the two sites overlap);

2. Although the repressor was first thought to be i-gene RNA, isolating it proved that it is protein; and

3. Jacob and Monod proposed that all transcriptional regulation was negative; that is, that binding a regulatory protein always inhibited transcription. However, the lactose operon, like many other regulated genes, also exhibits positive control of transcription; that is, binding a protein activates transcription in certain cases, as described here.

1. The Lac Operon

2. Induction of the Lac Operon

3. Inducible Genes