operon lac

• The lac operon of E. coli contains genes involved in lactose metabolism. It's expressed only when lactose is present and glucose is absent.

• Two regulators turn the operon "on" and "off" in response to tướng lactose and glucose levels: the lac repressor and catabolite activator protein (CAP).

• The lac repressor acts as a lactose sensor. It normally blocks transcription of the operon, but stops acting as a repressor when lactose is present. The lac repressor senses lactose indirectly, through its isomer allolactose.

• Catabolite activator protein (CAP) acts as a glucose sensor. It activates transcription of the operon, but only when glucose levels are low. CAP senses glucose indirectly, through the "hunger signal" molecule cAMP.

Lactose: it's what's for dinner! While that may not sound delicious to tướng us (lactose is the main sugar in milk, and you probably don't want to tướng eat it plain), lactose can be an excellent meal for E. coli bacteria. However, they'll only gobble up lactose when other, better sugars – lượt thích glucose – are unavailable.

With that for context, what exactly is the lac operon? The lac operon is an operon, or group of genes with a single promoter (transcribed as a single mRNA). The genes in the operon encode proteins that allow the bacteria to tướng use lactose as an energy source.

E. coli bacteria can break down lactose, but it's not their favorite fuel. If glucose is around, they would much rather use that. Glucose requires fewer steps and less energy to tướng break down kêu ca lactose. However, if lactose is the only sugar available, the E. coli will go right ahead and use it as an energy source.

To use lactose, the bacteria must express the lac operon genes, which encode key enzymes for lactose uptake and metabolism. To be as efficient as possible, E. coli should express the lac operon only when two conditions are met:

• Lactose is available, and
• Glucose is not available

How are levels of lactose and glucose detected, and how how tự changes in levels affect lac operon transcription? Two regulatory proteins are involved:

• One, the lac repressor, acts as a lactose sensor.
• The other, catabolite activator protein (CAP), acts as a glucose sensor.

These proteins bind to tướng the DNA of the lac operon and regulate its transcription based on lactose and glucose levels. Let's take a look at how this works.

The lac operon contains three genes: lacZ, lacY, and lacA. These genes are transcribed as a single mRNA, under control of one promoter.

Genes in the lac operon specify proteins that help the cell utilize lactose. lacZ encodes an enzyme that splits lactose into monosaccharides (single-unit sugars) that can be fed into glycolysis. Similarly, lacY encodes a membrane-embedded transporter that helps bring lactose into the cell.

In addition to tướng the three genes, the lac operon also contains a number of regulatory DNA sequences. These are regions of DNA to tướng which particular regulatory proteins can bind, controlling transcription of the operon.

• The promoter is the binding site for RNA polymerase, the enzyme that performs transcription.

• The operator is a negative regulatory site bound by the lac repressor protein. The operator overlaps with the promoter, and when the lac repressor is bound, RNA polymerase cannot bind to tướng the promoter and start transcription.

• The CAP binding site is a positive regulatory site that is bound by catabolite activator protein (CAP). When CAP is bound to tướng this site, it promotes transcription by helping RNA polymerase bind to tướng the promoter.

Let's take a closer look at the lac repressor and CAP and their roles in regulation of the lac operon.

The lac repressor is a protein that represses (inhibits) transcription of the lac operon. It does this by binding to tướng the operator, which partially overlaps with the promoter. When bound, the lac repressor gets in RNA polymerase's way and keeps it from transcribing the operon.

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When lactose is not available, the lac repressor binds tightly to tướng the operator, preventing transcription by RNA polymerase. However, when lactose is present, the lac repressor loses its ability to tướng bind DNA. It floats off the operator, clearing the way for RNA polymerase to tướng transcribe the operon.

This change in the lac repressor is caused by the small molecule allolactose, an isomer (rearranged version) of lactose. When lactose is available, some molecules will be converted to tướng allolactose inside the cell. Allolactose binds to tướng the lac repressor and makes it change shape so sánh it can no longer bind DNA.

Allolactose is an example of an inducer, a small molecule that triggers expression of a ren or operon. The lac operon is considered an inducible operon because it is usually turned off (repressed), but can be turned on in the presence of the inducer allolactose.

When lactose is present, the lac repressor loses its DNA-binding ability. This clears the way for RNA polymerase to tướng bind to tướng the promoter and transcribe the lac operon. That sounds lượt thích the over of the story, right?

Well...not quite. As it turns out, RNA polymerase alone does not bind very well to tướng the lac operon promoter. It might make a few transcripts, but it won't tự much more unless it gets extra help from catabolite activator protein (CAP). CAP binds to tướng a region of DNA just before the lac operon promoter and helps RNA polymerase attach to tướng the promoter, driving high levels of transcription.

CAP isn't always active (able to tướng bind DNA). Instead, it's regulated by a small molecule called cyclic AMP (cAMP). cAMP is a "hunger signal" made by E. coli when glucose levels are low. cAMP binds to tướng CAP, changing its shape and making it able to tướng bind DNA and promote transcription. Without cAMP, CAP cannot bind DNA and is inactive.

CAP is only active when glucose levels are low (cAMP levels are high). Thus, the lac operon can only be transcribed at high levels when glucose is absent. This strategy ensures that bacteria only turn on the lac operon and start using lactose after they have used up all of the preferred energy source (glucose).

The lac operon will be expressed at high levels if two conditions are met:

• Glucose must be unavailable: When glucose is unavailable, cAMP binds to tướng CAP, making CAP able to tướng bind DNA. Bound CAP helps RNA polymerase attach to tướng the lac operon promoter.

• Lactose must be available: If lactose is available, the lac repressor will be released from the operator (by binding of allolactose). This allows RNA polymerase to tướng move forward on the DNA and transcribe the operon.

These two events in combination – the binding of the activator and the release of the repressor – allow RNA polymerase to tướng bind strongly to tướng the promoter and give it a clear path for transcription. They lead to tướng strong transcription of the lac operon and production of enzymes needed for lactose utilization.

Now that we’ve seen all the moving parts of the lac operon, let’s put what we’ve learned together to tướng see how the operon reacts to tướng a variety of different conditions (presence or absence of glucose and lactose).

• Glucose present, lactose absent: No transcription of the lac operon occurs. That's because the lac repressor remains bound to tướng the operator and prevents transcription by RNA polymerase. Also, cAMP levels are low because glucose levels are high, so sánh CAP is inactive and cannot bind DNA.

• Glucose present, lactose present: Low-level transcription of the lac operon occurs. The lac repressor is released from the operator because the inducer (allolactose) is present. cAMP levels, however, are low because glucose is present. Thus, CAP remains inactive and cannot bind to tướng DNA, so sánh transcription only occurs at a low, leaky level.

• Glucose absent, lactose absent: No transcription of the lac operon occurs. cAMP levels are high because glucose levels are low, so sánh CAP is active and will be bound to tướng the DNA. However, the lac repressor will also be bound to tướng the operator (due to tướng the absence of allolactose), acting as a roadblock to tướng RNA polymerase and preventing transcription.

• Glucose absent, lactose present: Strong transcription of the lac operon occurs. The lac repressor is released from the operator because the inducer (allolactose) is present. cAMP levels are high because glucose is absent, so sánh CAP is active and bound to tướng the DNA. CAP helps RNA polymerase bind to tướng the promoter, permitting high levels of transcription.

LabXchange is a không tính tiền online science education platform created at Harvard’s Faculty of Arts and Sciences and supported by the Amgen Foundation.

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