Lac operon

The lac operon consists of three adjacent genes required for the transport and of lactose (milk sugar) in the Escherichia coli (E. coli) and some other bacteria. The term operon is used when genes (in this case lacZYA) are co-transcribed into a single messenger RNA. The lac operon is regulated by several factors, one of which is the availability of lactose as an energy source. Control of the lac genes was the first genetic regulatory mechanism to be elucidated, one reason for this is that it is one of the simplest, at least in outline, consisting of simple negative (lac repressor) and positive (CAP) regulatory elements. The lac operon has been considered the canonical example of prokaryotic gene regulation.

Regulation by cyclic AMP

The experimental microorganism used by François Jacob and Jacques Monod was the common laboratory bacterium, E. coli, but many of the basic regulatory concepts (described below) that were discovered by Jacob and Monod are fundamental to cellular regulation in organisms. The key idea is that E. coli conserves cellular resources and energy by not making the three Lac proteins when there is no need to metabolize lactose, such as when other sugars like glucose are available. The key question was 'how does E. coli control certain genes in response to metabolic needs?

Related Topics:
Microorganism - François Jacob - Jacques Monod - Cell - Glucose

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During World War II, Monod was testing the effects of combinations of sugars as nutrient sources for E. coli. He found that bacteria grown with two different sugars often displayed two phases of growth. For example, if glucose and lactose were both provided, glucose would be metabolized first (growth phase I, see Figure 2) and then lactose (growth phase II). This phenomenon is called diauxy.

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Metabolism of lactose does not occur during the first part of the diauxic growth curve because ?-galactosidase is not made when both glucose and lactose are present in the medium.

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Explanation of this depended on the characterization of additional mutations affecting the lac genes other than those explained by the classical model. Two other genes were known that map far away from lac, and result in a decreased level of expression in the presence of IPTG and even in a strain which is mutant for repressor or operator. The discovery of cyclic AMP in 1967 (in eukaryotic cells) led to the demonstration that mutants defective in one of these genes could be restored to full activity by the addition of cyclic AMP to the medium. The cya gene encodes adenylate cyclase, which produces cyclic AMP. In a cya mutant, the absence of cyclic AMP makes the expression of the lacZYA genes somewhat lower than normal. Addition of cyclic AMP corrects the low Lac expression characteristic of cya mutants. The second gene, crp, encodes a protein called catabolite activator protein (CAP) or catabolite repressor protein (CRP).

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This dual regulation causes the lactose metabolism enzymes to be made in small quantities in the presence of glucose and lactose (sometimes called leaky expression) due to the lactose inhibiting LacI binding of the operator, but at high cAMP concentrations and in the presence of lactose there are high levels of expression (Phase II in Figure 2). Leaky expression is necessary in order to allow for metabolism of some lactose after the glucose source is expended, but before lac expression is fully activated.

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In summary:

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• When lactose is absent then there is no Lac enzyme production (the operator has LacI bound to it).
• When lactose is present but a prefered carbon source (like glucose) is also present then a small amount of enzyme is produced (LacI is not bound to the operator).
• When lactose is the favoured carbon source (for example in the absence of glucose) cAMP-CAP bind to the promoter and Lac enzyme production is maximised.

So why is there a delay between the two growth phases? First, the CAP regulatory protein has to assemble on the lac operator, resulting in an increase in the production of lac mRNA. More available copies of the lac mRNA results in the production (see translation) of significantly more copies of LacZ (β-galactosidase, for lactose metabolism) and LacY (lactose permease to transport lactose into the cell) are produced. After a delay needed to increase the level of the lactose metabolizing enzymes, the bacteria enter into a new rapid phase of cell growth.

Related Topics:
MRNA - Translation - Cell growth

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