Question

Why is the lac operon in inducible? (i.e. why does the cell normally keep it off?)

Answer 1

• 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 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.

  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. lacZencodes 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 the three genes, the lac operon also contains a number of regulatory DNA sequences. These are regions of DNA to 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 lacrepressor is bound, RNA polymerase cannot bind to 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 this site, it promotes transcription by helping RNA polymerase bind to the promoter

• The lac repressor is a protein that represses (inhibits) transcription of the lacoperon. It does this by binding to 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.

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

  Upper panel: No lactose. When lactose is absent, the lacrepressor binds tightly to the operator. It gets in RNA polymerase's way, preventing transcription.

  Lower panel: With lactose. Allolactose (rearranged lactose) binds to the lacrepressor and makes it let go of the operator. RNA polymerase can now 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 allolactose inside the cell. Allolactose binds to the lac repressor and makes it change shape so it can no longer bind DNA.

  Allolactose is an example of an inducer, a small molecule that triggers expression of a gene 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.

  Catabolite activator protein (CAP)

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

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

  Upper panel: Low glucose. When glucose levels are low, cAMP is produced. The cAMP attaches to CAP, allowing it to bind DNA. CAP helps RNA polymerase bind to the promoter, resulting in high levels of transcription.

  Lower panel: High glucose. When glucose levels are high, no cAMP is made. CAP cannot bind DNA without cAMP, so transcription occurs only at a low level.

  CAP isn't always active (able to bind DNA). Instead, it's regulated by a small molecule called cyclic AMP (cAMP). cAMP is a "hunger signal" made by E. coliwhen glucose levels are low. cAMP binds to CAP, changing its shape and making it able to 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).

  So, when does the lac operon really turn on?

  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 CAP, making CAP able to bind DNA. Bound CAP helps RNA polymerase attach to 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 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 bind strongly to the promoter and give it a clear path for transcription. They lead to 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 see how the operon reacts to 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 the operator and prevents transcription by RNA polymerase. Also, cAMP levels are low because glucose levels are high, so CAP is inactive and cannot bind DNA

• Glucose present, lactose present: Low-level transcription of the lacoperon occurs. The lacrepressor 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 DNA, so transcription only occurs at a low, leaky level.

• Summary of lac operon responses

  Glucose	Lactose	CAP binds	Repressor binds	Level of transcription
  +	-	-	+	No transcription
  +	+	-	-	Low-level transcription
  -	-	+	+	No transcription
  -	+	+	-	Strong transcription

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