Lac operon - Lecture notes PDF

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Lac operon

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With the lac operon, lac refers to lactose, which is a sugar found in milk, and an operon is a portion of DNA where genes with related functions are grouped together and are controlled by the same promoter.

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In the case of the lac operon, the proteins the lac operon produces are all required for transporting lactose into the cell and metabolising it in Escherichia coli as well as other bacteria.

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Although glucose is the preferred carbon source for most bacteria, the lac operon allows these bacteria to use lactose when glucose isn’t available.

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Gene regulation of the lac operon is well studied, and that’s why it has become a classic example of gene regulation in bacteria.

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Now before getting into the details of the lac operon and how it functions, let’s review gene expression.

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DNA is made up of genes, and each gene is basically a specific part of the DNA that codes for a protein.

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Genes become proteins in two steps: transcription and translation.

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In transcription a segment of DNA is copied into RNA, specifically messenger or mRNA, by the enzyme RNA polymerase.

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RNA polymerase unwinds the DNA double helix to produce the complementary mRNA, which is like a blueprint on what protein to build.

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Then there’s translation which is where organelles called ribosomes assemble and utilize the mRNA produced during transcription to create proteins from amino acids lying around in the cytoplasm.

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Now, the lac operon is a part of E coli’s DNA and it includes structural genes, like lacZ, lacY, and lacA, as well as regulatory genes like the promoter and operator.

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The structural genes lacZYA code for the proteins that ultimately allow E coli to transport and metabolize lactose.

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LacZ, produces the enzyme beta galactosidase, also called lactase, which break down lactose into glucose and galactose.

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LacY produces beta-galactosidase permease, which allows lactose to enter, or permeate into the cell, and lacA encodes beta-galactoside transacetylase, and its function isn’t clearly understood.

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Now, in addition to those structural genes, there’s the promoter and operator, which tell the operon when to start and stop transcription.

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The promoter is a nucleotide sequence on the DNA where RNA polymerase binds and begins transcribing mRNA.

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The operator, on the other hand, is located in between the promoter and the structural genes and it works together with the lac repressor protein which is encoded by the lacI gene - which is found upstream of the promoter.

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When there’s glucose available there’s no need to metabolize lactose, so the lac repressor protein binds to the operator region, and it acts like a bouncer, physically blocking RNA polymerase from attaching to and transcribing the lacZYA genes.

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So depending on the concentrations of glucose and lactose in the cell, E. coli can turn the lac operon genes on and off - like a light switch.

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During the good times, there’s plenty of glucose available for E. coli to use for energy, so it doesn’t need to produce lacZYA proteins.

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This happens even when there’s glucose and lactose around, because glucose metabolism is more efficient.

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So in that situation, the lac repressor stays bound to the operator blocking RNA polymerase, and the lac operon is turned off.

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However, when there’s very little glucose around, but lots of lactose present, the lac operon has to be turned on.

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The way that happens, is that some of the lactose naturally converts into its isomer, allolactose.

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Allolactose then binds to the lac repressor protein and changes its shape, so that it falls off of the operator region.

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With the lac repressor out of the way, RNA polymerase has a clear path and can move down the operon to transcribe the lacZYA genes, producing the proteins necessary to metabolize lactose.

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Once the lactose is digested, it’s concentration decreases and that lowers the levels of allolactose.

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Without allolactose, the repressor protein remains in its natural shape sticks to the operator, turning off the lac operon.

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Now, there’s one more hitch. See, RNA polymerase doesn’t bind that well to DNA, so it can’t do that good a job at making a lot of mRNA from the lacZYA genes.

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So it makes a few proteins, but not nearly enough.

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Luckily, there’s help from our friendly neighbourhood catabolite activator protein, or CAP.

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CAP activity is regulated by the concentration of glucose in the cell.

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When there’s very little glucose around, an enzyme called adenylate cyclase converts a lot of ATP into cyclic, or cAMP.

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cAMP binds to CAP and changes its shape, allowing CAP to then bind to a DNA sequence just upstream of the lac operon region.

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When CAP is bound, it helps RNA polymerase transcribe the lacZYA genes more effectively, thus increasing protein production, and lactose digestion.

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You can also think of CAP as a glucose sensor.

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When glucose levels are low, CAP helps drive more lactose protein transcription, and when glucose levels are high, CAP helps drive less lactose protein transcription....