Amanda Days
Professor Rinehart-Kim
Biol 294 Genetics
12/09/2022
“Genetics Topic Assignment: The Lac Operon and its Relation to Gene Expression”
Gene expression is the regulation of certain genes to control when and how much they are expressed. The Lac Operon is an example of how cells regulate gene expression through the use of an off and on switch, as to not waste materials when a substance is not needed. This operon is just one example of negative control within the body to regulate expression.
Gene expression is used to regulate when and what is made in the cell to maintain homeostasis. This has to be carefully regulated so the cell does not waste excess materials on substances that do not need to be synthesized. This regulation can occur at many different points in the transcription and translational processes. During transcription initiation, promoters and enhancers can influence the way Polymerases attach or transcribe mRNA. Even after transcription, many processes must take place before the mRNA is able to continue to Translation. Transcription, however, is the main place that regulation will occur in Eukaryotes because it is the control point for gene expression. This is due to the positive and negative regulation caused by activators and repressors that influence when and if polymerases can bind and/or transcribe the gene.
Activators help increase the production of a gene while repressors inhibit the transcription of one. The Lac Operon is an example of gene expression that is influenced or controlled by a repressor. Operons are the clusters of genes that are co-transcribed together meaning that if one of those genes are turned “on”, then they all will need to be turned “on”, and vice versa (Macauley et al., 2009). As stated in the review article by Matthew Macauley, Andy Jenkins, and Robin Davies, these operons can also be described as inducible or repressible, depending on the default mode that the operon is in. It was also stated in this article that these operons can also be described as positive or negative depending on whether it requires a repressor to prevent transcription or an activator to start it (Macauley et al., 2009). For example, the Lac Operon is an example of a negative form of gene expression because of its utilization of negative control and repressors. This meaning that the lack of lactose will allow the repressor to bond to the operator, preventing transcription of the gene.
The Lac Operon of E. coli regulates the transcription of the necessary molecules to break down lactose. This operon consists of the Lac A gene, Lac Y gene, and Lac Z gene which are all necessary for the catabolic process involving the breakdown of Lactose. This is why this operon is regulated by the presence of Lactose, specifically allolactose, which is an isomer of lactose. The Lac operon would be described as a negative controlled operon due to its use of repressors and is usually in the “On”, mode. This means that the gene is set up in a way that without the inhibition of a repressor, it will continue to transcribe the genes that it encodes for. An article by Charles Bell and Mitchell Lewis outlines the structure of the Lac Operon as well as its structure and function. They stated that this operon is a great system that has furthered the understanding of the gene regulation and protein- DNA interactions. The repressor used in this operon actually binds to three different sites in the Lac operon unless it is bound by allolactose (Bell & Lewis, 2001). When allolactose binds to the repressor, the repressor goes through an allosteric change which in a way changes its shape so it cannot bind to the operon. If the repressor cannot bind to the operon, it cannot block the polymerase, which will be able to synthesize the genes without interruption (Bell & Lewis, 2001).
The Lac operon is influenced by the presence of lactose as well as glucose. This is due to the preference of glucose to the use of lactose in E. coli and this specific pathway for regulation of gene expression takes place in the transcriptional phase. The absence of glucose causes the cAMP levels to increase and bind to CAP which makes it active. This change helps recruits RNA polymerase to the operon promoter to be used for synthesis. The absence of glucose, however, does not impact the operon’s ability to be transcribed. When glucose levels are low and cAMP and CAP bind to the operon, it only helps the polymerase bind to the structure which would increase the transcription levels, but not allow nor prevent them from being synthesized. In other words, when glucose is present, the “On/Off”, factor of the operon is still controlled by the presence of lactose. When lactose is present, the Lac Operon will be able to be transcribed, which is ideal because it transcribes the necessary biomolecules to break down lactose. The isomer of lactose, allolactose, will then bind to the repressor and cause an allosteric change, or a change in shape to the repressor. This prevents the repressor from binding to the operator of the Lac operon preventing transcription. This also means that in the absence of lactose, the lactose isomer will not bind to the repressor, allowing it to bind to the operator. This binding of the repressor to the operator prevents the Polymerase from being able to move down the cluster of genes to transcribe them, preventing the transcription process by physically blocking it. This would also occur if glucose and lactose were both absent, but in this scenario, the cAMP/CAP complex would also be bound to the operon.
In conclusion, gene expression is an organism’s way of regulating genes to control what and when biomolecules are being produced. This process is important because it maintains homeostasis and prevents the cell from wasting materials to synthesize unnecessary biomolecules that were not needed for that given time. Regulation can happen at any time in the transcription and translation process, but the main point focused in this paper was transcriptional. This is due to the use of repressors and activators which interacts with an operon like an “On/Off” switch to either stop the transcription or to start it, depending on the default configuration. An Operon is a cluster of genes that are co-transcribed, the example for this paper being the Lac Operon. The Lac Operon is a cluster of genes used to transcribe the necessary proteins to break down lactose. The allolactose molecule interacts with a repressor to prevent it from binding to the operator when the operon needs to be synthesized, which is in the presence of lactose. The binding of the repressor to allolactose causes an allosteric change that prevents the repressor from binding to the operator and in turn, blocking the polymerase. This is important because it is the cell’s way of preventing the transcription when lactose is not present because without it, the repressor will block the polymerase from transcribing the operon which is in the “On” configuration. The use of allolactose as the molecule to remove the repressor enables the regulation to be precise, preventing the waste of resources. This operon gives researchers a better understanding of how operons work in the body and how they could be influenced to give desired results.
Sources:
Bell, C. E. & Lewis, M. The Lac Repressor: a Second Generation of Structural and Functional Studies. Current Opinion in Structural Biology 11, 19–25 (2001).
Macauley, M., Jenkins, A. & Davies, R. Chapter 4 – The Regulation of Gene Expression by Operons and the Local Modeling Framework. Algebraic and Combinatorial Computational Biology 89–146 (2009). doi:https://doi.org/10.1016/B978-0-12-814066-6.00004-0
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