The lac operon is a group of structural genes whose function is to code for proteins involved in the metabolism of lactose. They are genes that are ordered consecutively in the genome of almost all bacteria and have been studied with special effort in the "model" bacterium Escherichia coli.
The lac operon was the model used by Jacob and Monod in 1961 for proposing genetic arrangements in the form of an operon. In their work, these authors described how the expression of one or more genes could be “turned on” or “turned off” as a consequence of the presence of a molecule (lactose, for example) in the growth medium.
General scheme of the lac operon. Tereseik. Derivative work of G3pro image. Spanish translation by Alejandro Porto.
Bacteria that grow in growth media rich in carbonaceous compounds or sugars other than lactose, such as glucose and galactose, possess very low amounts of the proteins necessary for the metabolization of lactose.
Then, in the absence of lactose, the operon is "turned off", preventing the RNA polymerase from transcribing the gene segment corresponding to the lac operon. When the cell “senses” the presence of lactose, the operon is activated and these genes are normally transcribed, which is known as “turning on” the operon.
All the genes of the operon are translated into a single molecule of messenger RNA and, therefore, any factor that regulates the transcription of this messenger RNA of the lac operon will directly regulate the transcription of any gene that belongs to it.
Discovery
The Jacob and Monod theory developed in a context where very little was known about the structure of DNA. And it is that only eight years before Watson and Crick had made their proposal on the structure of DNA and RNA, so that messenger RNAs were hardly known.
Jacob and Monod in the 1950s had already shown that bacterial lactose metabolism was genetically regulated by two very specific conditions: the presence and absence of lactose.
Both scientists had observed that a protein with characteristics similar to an allosteric enzyme was able to detect the presence of lactose in the medium and that once the sugar was detected, the transcription of two enzymes was stimulated: a lactose permease and a galactosidase.
Nowadays it is known that permease has functions in the transport of lactose into the cell and that galactosidase is necessary to “break” or “cut” the lactose molecule into glucose and galactose, so that the cell can take advantage of this disaccharide in its constituent parts.
By the 1960s it had already been determined that lactose permease and galactosidase were encoded by two adjacent genetic sequences, the Z region and the Y region, respectively.
Lac operon is part of the genome of the bacterium Escherichia coli. Source: NIAID, via Wikimedia Commons
Finally, in 1961, Jacob and Monod presented a genetic model made up of five genetic elements:
- A promoter
- An operator and
- genes Z, Y and A.
All these segments are translated into a single messenger RNA and comprise the essential parts to define practically any bacterial operon in nature.
Genetic analysis and experiments
Jacob, Monod, and their collaborators conducted many experiments with bacterial cells that had mutations that rendered the strains unable to metabolize lactose. Such strains were identified by the name of the strain and the corresponding mutation they possessed.
In this way, the researchers were able to identify that mutations in the genes lacZ, which codes for β-galactosidase, and lacY, which codes for lactose permease, produced bacteria of the lac - type, that is, bacteria incapable of metabolizing lactose..
From “genetic mapping” using restriction enzymes, the location of the genes in the different strains was subsequently determined, a fact that allowed establishing that the three genes lacZ, lacY and lacA are found (in that order) on the bacterial chromosome in a group of adjacent genes.
The existence of another protein, called the repressor protein, which is not necessarily considered as "part" of the operon, was elucidated through mutations in a gene called lacI-. It encodes a protein that binds to the "operator" region in the operon and prevents transcription of the genes for β-galactosidase and lactose permease.
It is said that this protein is not part of the genes that make up the lac operon, since they are actually located "upstream" of the latter and are transcribed into different messenger RNAs.
Schematic of the lac operon (Source: Barbarossa at Dutch Wikipedia via Wikimedia Commons)
Bacterial strains that possess the lacI- mutation "constitutively" express the lacZ, lacY and lacA genes, which occurs regardless of the presence or absence of lactose in the extracellular environment.
Many of these observations were corroborated by transferring the lacI + and lacZ + genes to a bacterial cell that did not produce the proteins encoded by these genes in a lactose-free medium.
Since bacteria "transformed" in this way only produced the enzyme β-galactosidase in the presence of lactose, the experiment confirmed that the lacI gene was important for the regulation of lac operon expression.
Function
The lac operon regulates the transcription of genes that are necessary for bacteria to assimilate lactose as a source of carbon and energy. However, the transcription of these genes only occurs when the main energy source corresponds to galactoside-type carbohydrates.
In bacterial cells, there are mechanisms that regulate the expression of the lac operon genes when they are in the presence of glucose or any other sugar that is easier to metabolize.
The metabolization of these sugars involves their transport to the interior of the cell and their subsequent breakdown or processing.
Lactose is used as an alternative energy source for bacteria, helping them survive even after other energy sources in the environment such as glucose are depleted.
The lac operon model was the first genetic system of its kind to be elucidated and thus served as the basis for describing many other operons in the genome of different types of microorganisms.
With the study of this system, much progress was made in understanding the functioning of the "repressor" type proteins that bind to DNA. There was also progress in the understanding of allosteric enzymes and how they act selectively when recognizing one or another substrate.
Another important advance that arose from the study of the lac operon was the establishment of the crucial role that messenger RNAs play in translating the instructions found in DNA and also as a preliminary step to protein synthesis.
References
- Griffiths, AJ, Wessler, SR, Lewontin, RC, Gelbart, WM, Suzuki, DT, & Miller, JH (2005). An introduction to genetic analysis. Macmillan.
- Hartwell, L., Goldberg, ML, Fischer, JA, Hood, LE, & Aquadro, CF (2008). Genetics: from genes to genomes (pp. 978-0073227382). New York: McGraw-Hill.
- Lewis, M. (2013). Allostery and the lac Operon. Journal of molecular biology, 425 (13), 2309-2316.
- Müller-Hill, B., & Oehler, S. (1996). The lac operon (pp. 66-67). New York:: Walter de Gruyter.
- Parker, J. (2001). lac Operon.
- Yildirim, N., & Kazanci, C. (2011). Deterministic and stochastic simulation and analysis of biochemical reaction networks: The lactose operon example. In Methods in enzymology (Vol. 487, pp. 371-395). Academic Press.